Physics Flashcards
Terminology to characterise an atom?
A/Z(X) where A is number of nucleons, Z protons, X chemical symbol
The atomic number and mass numbers are denoted by which characters?
An element is defined by?
Atomic number = Z the number of protons
Mass number = A proton + neutrons
Element defined by Z, A defined the isotopes of X.
The nuclear binding energy means the mass of A is less than the sum of its parts
In the Bohr model each shell can hold how many electrons?
2(n^2) where n is the shell number
1st shell = k = 2
2nd = l = 8
Electron binding energy is the energy that be supplied to remove an electron from its shell.
Elevating an electron from K to l requires?
An amount of energy equal to the difference in binding energies
E>Ek
Define the electron volt
eV is the energy gained by an electron as it is accelerated by a potential difference of 1 volt
Characteristic radiation occurs when? Under what condition can this process be a cascade?
As a consequence is this emission continuous or discrete?
An outer shell electron falling to a more proximal shell - requires a vacancy.
High atomic number atoms have many shells and so the loss of a proximal election will trigger a cascade of electrons falling from higher shells. Each releasing a discrete amount of energy proportional to the binding energy of their shell.
Define radioactive decay and radioactivity?
Instability in ratio of neutrons to proton leads to a transition to a more stable configuration, this change emits particles and EMR with the energy corresponding to the increase in binding energy at final configuration. The change is referred to as decay, the emission as radioactivity.
In a sample of radionuclide the rate of decay is directly proportional to:
the number of atoms of the radionuclide present. A these change in the their progeny nuclide the rate decreases - is a decaying exponential function of initial number of atoms and a decay constant (ln2/T half)
Sometime used to determine dose delivery from permanent brachy implant Average life or mean life is defined as
1/the decay constant, where the decay constant is ln2/over half life = lamda). ie. 1/lamda = 1.44(T half)
Planck’s constant
h=6.62 x 10^-34 J.sec
Relate the energy of a photon to its wavelength
E=hv, where h is Planck’s and v frequency, where v=speed of light (c)/wavelength (i.e wavelength x v = c)
Production of gamma rays
By decay or capture. Decay can be either (or a mix of both - see decay schemes) betaminus (electron emitted by a neuron turning into a positron), or betaplus (when proton and electron combine to make a neutron).
In capture - inner orbital electron captured to make neutron from positron, also releases characteristic radiation (with possible cascade).
Principle elements of an Xray tube
1) Filament (cathode) - releases electron by thermionic emission
2) Target - Large atomic nuclei such as tungsten
3) Anode - At the site of the target,
4) High Voltage between cathode and anode accelerates electron onto focal spot (determined by size of filament) on target
In Kv imaging,what is the focal spot? How is it changed? What is the effect of shrinking the focal spot?
The focal spot is the site where accelerated electrons strike the target. The size of the filament/cathode determines the size of the focal spot (modern systems often have 2 or more filaments).
Smaller focal spots result in a smaller region from which Xrs are emitted (the apparent source) producing a more detailed picture. To produce sharp images, focal spots need to be small but able to withstand heat loading without melting the anode target. A small focal spot is used when spatial resolution is important, while a large focal spot is employed when a short exposure time is the priority.
The possible outcomes of XR/Gamma ray interaction w/matter?
Photon may be 1) Scattered 2) Absorbed 3) Transmitted without interaction Photons that traverse the medium uninterrupted are called primary radiation. Those that are scattered or absorbed are termed attenuated.
The amount by which a photon beam is attenuated is a characteristic of:
1) Attenuating material
2) Photon beam spectrum
What is HVL
Half value layer, in units of cm or mm is the thickness of the attenuating material that reduces the intensity of a radiation beam to half its original value. Sometimes measured in air kerma rate.
Under conditions of good geometry a monoenergetic beam will be attenuated ______ with increasing thickness of the absorber
Exponentially
A polyenergetic beam is not attenuated _____ as the absorber preferentially absorbs _______
Exponentially
Low energy photons
True or False: In the case of a polyenergetic beam the second HVL is equal to or less than the 1st HVL
False. to reduce from 1/2 to 1/4 A much larger HVL may be needed to attenuate high energy photons.
Define the linear attenuation coefficient u
u = 0.693/HVL
The transmitted beam intensity I for any thickness of absorbing material:
I transmitted = I0^-ux where I0 = initial beam intensity u = linear attenuation coeef 0.693/HVL x is thickness u is in units cm^-1
Define Coulomb
What is the charge of the following particles: Proton Neutron Election Positron
1 Columb = it is the charge carried by a current of 1ampres in 1 second. Proton = 1.602 x10^-19C neutron = 0 Electron = -1.602 x10^-19 C Positron = +1.602 x10^-19
Rest mass of an electron
9.109 x10^-31 hbpKg = 0.511 MeV
If atoms are described in terms of A, Z, X What is Z What is the Neutron number? what is the notation for: Proton Neutron Positron Electron
Z= number of protons (and electrons)
N = A-Z
Proton: a=1 (i.e. n+p), Z=1 (i.e 1 proton/electron) symbol lower case p (A/P p)
Neuton: A = 1, Z=0, symbol n
Electron, A=0, Z=-1, symbol = e
Positron, A=0, Z=-1, symbol = e (same as electron)
Contrast Atomic mass and Atomic weight
Atomic mass (ma) is the mass of an atom. A single atom has a set number of protons and neutrons, so the mass is unequivocal (won’t change) and is the sum of the number of protons and neutrons in the atom. Electrons contribute so little mass that they aren’t counted.
Atomic weight is a weighted average of the mass of all the atoms of an element, based on the abundance of isotopes.
Both rely on the atomic mass unit (amu), which is 1/12th the mass of an atom of carbon-12 in its ground state
Define isotope
An atomic species with the same number of proton (atomic number Z) but different number of neutrons (mass number A)
Define energy
The ability to do work - in units of J (Kg.m^2/(s^2))
Work = Force x distance (Nm = J)
Force = any of the known forces (e.g strong, weak, electromagnetic) that causes an object to undergo change. (units of N = Kg.m/(s^2))
Define Force
Force = any of the known forces (e.g strong, weak, electromagnetic) that causes an object to undergo change. (units of N = Kg.m/(s^2))
Define kinetic energy
Kinetic energy = 1/2(mv^2)
The work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes.
Electromagnetic radiation (EMR) is made up of
AN electric field and magnetic field propagating through space
All EMR travels at the speed of light c in a vacuum. How does this relate to the frequency and wavelength of EMR
Where c is constant, lama = wavelength
C= f.lamda,
f=C/lamda
ect
A photon has energy E =
E photon = Planck’s (h) x frequency
(ie. f= c/wavelength, so E = h x (c/wavelength
A 25MeV Photon which undergoes Pair Production is an example of an ……… collision. The outcome of which produces
Inelastic collision - where some kinetic force is transferred into a different of energy.
In this case a positron and electron pair - each with mass 0.511 Mev and kinetic energy of (25 - 2(0.511) ) /2= (23.78)/2= 11.9Mev
A photon may interact with an atom either at the orbital electrons or nucleus. At each site list the type of interaction that may occur:
Photon-Orbital (in order of low to high energy): 1) Coherent Scattering 2) Photoelectric effect 3) Compton (Incoherent) Scattering Photon-Nucleus: 1) Pair Production 2) Photdisintegration
Coherent scattering is proportional to what?
When would scatter be highest
Energy of photon and atomic number (squared)
Scatter proportional to (Z^2)/E
i.e highest scatter for high atomic number (high electron density) material hit with lower energy beams
All electrons are not equally attractive to a photon. What makes an electron more or less attractive is its binding energy. The two general rules are:
- Photoelectric interactions occur most frequently when the electron binding energy is slightly less than the photon energy.
- Compton interactions occur most frequently with electrons with relatively low binding energies.
The total attenuation rate depends on the individual rates associated with
photoelectric and Compton interactions.
Utotal = Ucompton + Uphoto
kV beams produced with ….. target
MV beams produced with ….. target
kV beams produced with reflective target
MV beams produced with transmission target
In XR production what is the heel effect, what type of target creates them/beam strength
Seen in kV beams (i.e produced with reflective target):
Heel effect- beam intensity lower on the anode side due to increased anode material needed to pass through. ‘self attenuation’
In Mv photon production:
Mean energy production is 1/? less than nominal
Mean energy ⅓ of nominal energy
In XR production, for a tungsten target X% is characteristic, Y% bremsstrahlung
For tungsten target 20% characteristic, 80% bremsstrahlung
Inverse Square Law
Inverse Square Law
● beam diverges as it moves from its source
● intensity is inversely proportional to the square of the distance
The vast majority of Mv photons from a linear accelerator are produced from
Mostly bremsstrahlung
Beta minus decay:
Electron emitted by a proton turning into a neutron
Boom there’s your minus
Beta plus decay:
A proton turning into a neutron by combing with an electron
Electron capture
When a positron captures an inner shell electron to make a neutron, triggering characteristic radiation
The photoelectric effect requires a photon to interact with
A tightly bound inner orbital electron
The photoelectric effect dominates at?
low energies = less than 0.2 MeV
In the photoelectric effect, the maximum energy an electron can receive in any one interaction is
hv
Letters denoting the 1st 4 orbital shells from inner to outer
K, L, M, N
Photoelectric interactions usually occur with electrons that are ………. bound to the atom, that is, those with a relatively …….. binding energy.
Photoelectric interactions are most probable when?
Photoelectric interactions usually occur with electrons that are firmly bound to the atom, that is, those with a relatively high binding energy.
Photoelectric interactions are most probable when the electron binding energy is only slightly less than the energy of the photon.
Think K edge!!
Describe the photoelectric effect or draw a diagram of it
Photon (typically with energy only slightly more than the binding energy of inner shell electron is) is absorbed by inner shell electron removing from the atom with energy = hv-Ebinding.
Subsequently outer shell electrons (if present) will fill inner shell vacancies, emitting photons (characteristic XRs) with energies = to the difference between new inner orbital shell and previous orbital binding energies.
A further electron may be ejected from the atom if a characteristic XR with sufficient hv generated from the above process interacts with a loosely bound outer shell electron (Ebinding < hv) removing from the atom.
This electron is the Auger electron
How do Auger electrons occur?
An electron may be ejected from the atom if a characteristic XR with sufficient hv generated from the photoelectric effect interacts with a loosely bound outer shell electron (Ebinding < hv) removing from the atom.
This electron is the Auger electron
The probability of the photoelectric effect is dependent on?
Interaction most often in the …. shell? Because?
Z and E - with Z being much more important
Probability proportional to (Z^3) and inversely proportional to E.
Most Photoelectric interactions occur in the K shell because the density of the electron cloud is greater in this region and a higher probability of interaction exists. About 30% of photons absorbed from a clinical x-ray beam are absorbed by the photoelectric process.
Contrast the site of interaction between the photoelectric and compton effects
PE = inner shell (usually k) Compton = loosely bound outer shell electron (no Auger's produced)
The probability of compton interactions is largely dependent on ……
Since most materials are similar in this regard, attenuation in units of (….) is the same, but differs in
Electron density - which is approximately the same for most atoms.
Therefore in terms of grams/cm^3 compton attenuation is the same for most materials.
BUT attenuation per cm is different.
1cm of bone has more electrons/cm^3 than 1cm of soft tissue
The K-edge is a sudden increase in
The K-edge is a sudden increase in XR absorption/attenuation as the photon energy just exceeds the inner shell binding energy such that photoelectric absorption of the photon is more likely to occur.
Note: there are also L3,2,1 edges at very low energies (around 10KeV)
The threshold for pair production is:
Why?
What happens to the excess energy?
It is due to a photon interaction with
1.022Mev (remember this number!)
Because an electron has a rest mass equivalent to 0.511 MeV of energy, a minimum gamma-energy of 1.02 MeV is required to produce 2. Any excess energy of the pair-producing gamma-ray is given to the electron–positron pair as kinetic energy. i.e. hv-1.022 MeV
It is due to a photon interaction with the coulomb field of the nucleus
Mass attenuation from photo electric effect is dependent on:
Z - i.e. the bigger the nucleus column field the more likely
What is fate of the products of pair production?
Electron - undergoes electron interactions/energy transfer
Positron - ANNIHILATION reaction with an electron - creating 2 photons each with hv = 0.511 = (the rest mass of the positron + the electron it collided with)/2.
The 2 types of photon interactions with nuclei in order from low to high of energies involved
1) Pair production - threshold is 1.022 Mev (2x the rest mass of an electron)
2) Photodisintergration - above 10MeV
Photodisintegration occurs above?
10MeV
Photo disintegration is an interaction between a high energy photon and …………… It results in:
Photo disintegration is an interaction between a high energy photon and an atomic nucleus.
It results in: the emission of 1 or more nucleons (typically a neutron), i.e leads to unstable rations and radioactivity.
For photodisintegration to occur threshold energy is:
The rest energy of nucleus - (rest energy residual nucleus + emitted nucleon)
If you were to graph attenuation in terms of which photon interaction with matter contributed at a given energy how would you plot it?
x-axis: Energy (KeV) on log10 10, 100, 1000, 10,000
y axis: Linear attenuation (cm^-1) 100, 1, 0.01, 0.0001
P.electric start at 100cm^-1 finish at 100KeV, K-edge at 10KeV
Compton: Tubby curve crossing y axis at 1, slow decrease to 1MeV then nonlinear drop to 10MeV
PP: starts at 1.022Mev nonlinear approach to flatten just over 0.01/cm at a bit past 10MeV
Show total attenuation curve whichh overlies the above curves
What would be the key components if you were to graph the relationship between atomic number and which photon interactions dominate at a particular energy.
Y-Axis: Zeff (effective proton number), from 0 to 100
X: Energy Log(MeV), 0.1, 1, 10, 100
PE: line where SIGMApe = SIGMAcompton = right half of Paraboler asymtope at Zef =100, MeV=1
Photon Energy is imparted to matter in a two stage process:
What energy is not counted as absorbed energy?
Thus the equation for average energy absorbed is:
Energy is imparted to matter in a two stage process:
● when a photon interacts with material, all or part of its energy is transferred into kinetic energy of electrons.
● most of these electrons lose their energy by inelastic collisions (ionization and excitation) with atomic electrons.
● A few will lose energy by bremsstrahlung interactions with the nuclei.
● Bremsstrahlung energy leaves the local volume as x-rays and is not included in the calculation of absorbed energy.
▁Eab= ▁Etr - ▁Erad (note: line should be under letters)
▁Etr is the average energy transferred from the primary photon to kinetic energy of charged particles
▁Erad is the average energy which the charged particles lose to bremsstrahlung and is not absorbed in the volume.
Define the linear attenuation coefficient
mu = fraction of absorbed photons per unit path length (cm)
Therefore in units of cm^-1
It is constant for monoenergetic beams (ie. takes the same form as half life).
For a beam of intensity Io, the exponentially decaying intensity observed at I(x cm) is described by:
I(x)= Io.e^(-mu.x)
The linear attenuation coefficient is dependent on?
To remove this dependency, such that it only reflects Z and beam intensity, what can be done?
Dependent on beam intensity, and density.
Divide by density, leaving the atomic number and initial beam intensity (Izero) as independent variables.
symbol mu/p
I.e cm^-1/(g.cm^-3) = (cm^2).g^-1
Units for mass attenuation
(cm^2).g^-1
i.e linear attenuation/density, cm^-1/(g.cm^-3)
mass attenuation is dependent on
Z and beam intensity
Electronic and atomic attenuation are derived from?
What are their units
Both atomic and electronic attenuation are derived from mass attenuation (u/d). With units of electrons/cm2 and atoms/cm2
Where Nzero is the number of electrons/gram: eU = (u/d)*1/Nzero
units: electrons/cm2
aU = (u/d)/(Z/No) (units atoms/cm2)
How would you derive the linear attenuation from mass attenuation
If mass attenuation is linear attenuation divided by density, then you can calculate linear attenuation by multiplication if you know the density of the absorber in question.
mu = (mu/p)*p
Describe the Linear Energy Transfer Coefficient:
The LETC is the fraction of energy that is transferred to kinetic energy per unit thickness of the absorber. Using the ratio of the average amount of energy transferred per interaction to the amount of energy (_Etr/hv) which is then scaled to the amount of attenuation:
Utr = U*(_Etr/hv)
Describe the Linear Energy Absorption Coefficient
Fraction of PHOTON energy that is absorbed (converted to inelastic interactions of secondary particles) per unit thickness of the absorber.
If Utr is the attenuation of kinetic energy, then the energy absorption coeffiecient differs in that it captures what happens to this kinetic energy by removing the energy lost by secondary charged particles as bremsstrahlung (B).
If g is the fraction of energy lost as bremsstrahlung 1-g = the remaining energy.
Therefore Uenergy absorbed = (Fraction of energy converted to kinetic energy)*Fraction not lost to B by secondary particles.
Uen = Utr(1-g)
Derive the mass energy transfer coefficient:
Linear energy transfer coefficient/density.
Under what conditions does Utr = Uen?
When Z is low, electrons lose most of their energy by ionisation collisions (very little is lost due to bremsstrahlung).
Therefore the fraction energy transferred to kinetic energy per unit thickness = fraction of energy lost to inelastic collisions with no secondary electons loosing energy to Brem:
U*(_Etr/hv)=Utr(1-g) IF g =0,
Hence,
Utr=Utr x ~1
Attenuation of a photon beam by an absorbing material is depenent on the 5 major types of interactions. Each associated with an attenuation coefficient which vary with?
Beam strength and the atomic number of the absorber.
Electrons undergo large number of ………. interactions before losing all their energy. They are also easily scattered due to their low …….
Electrons undergo large number of Coulomb interactions before losing all their energy. They are also easily scattered due to their low mass.
Electron Inelastic collisions:
○ with atomic electrons resulting in?
○ with atomic nuclei resulting in?
Typical energy loss is about ? in water
● Inelastic collision:
○ with atomic electrons resulting in ionization and excitation (collisional loss)
○ with atomic nuclei resulting in bremsstrahlung (radiative loss)
● Typical energy loss is about 2 MeV/cm in water
Electron Elastic collisions result in:
Elastic collisions resulting in scattering without energy loss
The two forms of electron interactions with matter are described by:
Scatter power
Stopping power
What is stopping power?
What does it depend on?
The retarding FORCE acting on CHARGED particles (typically alpha or beta)
The stopping power depends on the type and energy of the radiation and on the properties of the material it passes.
The stopping power of a material is numerically equal to:
The stopping power of the material is numerically equal to the loss of energy E per unit path length, x
S(E)=dE/dx
This eqn is the linear stopping power which in the international system is in N but is usually indicated in other units like MeV/cm
The range of a charged particle:
In passing through matter, charged particles ionize and thus lose energy in many steps, until their energy is (almost) zero. The distance to this point is called the range of the particle.
The range depends on: type of particle, its initial energy and the material through which it passes.
How is Stopping power graphed?
Bragg curve:
X axis: Path length (cm) e.g 0 - 4cm
Y axis: Force (Stopping power)
From a clinical point of view, what is the key feature of a Bragg curve?
What is the explanation for this feature
The Bragg peak.
Force significantly increases towards the end of the particle’s range.
This is due to the slowing particle interacting more with the medium through which it passes (increased cross-sectional area). Energy lost by charged particles is inversely proportional to the square of their velocity, which explains the peak occurring just before the particle comes to a complete stop.
In terms of an absorber, electrons/gram differ between low an high Z material how?
What are the implications for the type of Energy lost by an electron moving through a low, or high Z medium?
High Z materials have less electrons/gram and more tightly bound electrons
Low Z materials have more electrons/gram, which are less tightly bound leading to excitation and ionisation/
Electrons, moving through a material are more likely to have collisions with other electrons in Low Z materials. Whereas heavy atoms are more likely to cause braking radiation.
What is Mass Stopping Power?
What are its units?
Linear Stopping Power/Density (s/d)
S(E)=dE/dX in units of MeV/cm
Density in Gram/cm3
Therefore units of MeV.cm2/gram
For a solid medium, total mass stopping power is the sum of?
(s/d)tot=(s/d)collisional + (s/d)radiation
Where collisions leads to ionisation and excitation
Collisional stopping power is dependent on?
Energy bellow ~ 1.02MeV and Electrons per/gram, that are in turn weakly bound. Therefore dependent on Z:
(S)col proportional to 1/Z
(S) col proportional to 1/Esqr after 1.02Mev Becomes constant at around 2 MeV/gram.cm^-2
Radiation Stopping Power is dependent on?
What does this imply for XR production?
Z and E
(S)rad proportional to Z^2
(S)rad proportional to E
This implies the generation of bremsstahlung radiation is more efficient at high Z and E.
What is Delta radiation? What is another name for it?
Aka “Knock on electrons” are elections knocked out of their orbits by primary particles with sufficient energy to ionise other atoms.
Appears as branches off the main particle track in a cloud chamber
Total collisional stopping power is equal to?
Hard collisional and soft collisional stopping power
Therefore, if
(S)total = (S)collisional + (S)rad
(S)total = ((S)hard +(S)soft) + (S)rad
In terms of collisional stopping power. What are soft and hard collisions?
Soft (aka distant collisions): when the particle “collision” is far from the atom, the particle interacts weakly with all the orbital electrons - transferring relatively little E to each. But the large number of interaction entail the particle loses a lot of E (~50%)
Hard (Aka Close collisions). Particle interacts with an orbital electron transferring lots of energy to it - the electron turning into delta radiation. The chances of a hard collision are relatively small, but energy transferred is high - particles lose approx 50% of their Ek.
Restricted mass collision stopping power is introduced to calculate the energy transferred to?
By limiting the energy transferred to a ……… ……….. to a threshold (often denoted as D), highly energetic …… ……….. are allowed to escape the region of interest.
The restricted stopping power is lower than the unrestricted stopping power. The choice of the energy threshold depends on the problem at hand. For problems involving ionization chambers a frequently used threshold value is 10 keV (the range of a 10 keV electron in air is of the order of 2 mm).
Restricted mass collision stopping power is introduced to calculate the energy transferred to a localized region of interest.
By limiting the energy transfer to delta particles to a threshold (often denoted as D), highly energetic delta particles are allowed to escape the region of interest (and the calculation).
(S)RLMASS = (S)collisional - Edelta>threshold
The restricted stopping power is lower than the unrestricted stopping power. The choice of the energy threshold depends on the problem at hand. For problems involving ionization chambers a frequently used threshold value is 10 keV (the range of a 10 keV electron in air is of the order of 2 mm).
Define scatter power:
The scattering angle of a charged particle per unit path length within the absorber.
T= change in angle^2 per path length
Units rad^2/cm
Electron scattering depends on?
How?
E and Z
Z: Larger nuclei = large coulomb force = more scatter
Scatter proportional to Z^2
E: Higher the energy the less time near the nucleus therefore the less scatter.
Scatter proportional to 1/E^2
Define mass scattering power
Scatter power/density
(rad^2/cm)/(gram/cm^3) = rad^2.cm^2/gram
There are two major sources of gamma rays.
in one of these a very specific amount of gamma ray energy is produced…
One way in which gamma rays are emitted occurs when a nucleus moves from an excited energy level to a lower energy level. Energy from the transition is given off in the form of gamma rays.
The other way gamma rays are produced results from the annihilation of positron electron pairs. When this event occurs, two gamma rays, both of energy 511 keV, are emitted (i.e the energy equivalent of the rest mass of the colliding particles)
Define radiant energy
for monoenergetic beams
Is the number of particle emitted/transferred/received ect multiplied by their energies E.
R= NxE (units of joules)
E.g Energy fluence = R/dA = particle fluence x Energy of those particles (dN x Energy/dA)
Define flux:
for monoenergetic beams
Particle flux = Number of particles counted (dN)/time interval (dT) (units num/time)
Energy flux = Energy of parties (i.e N particles x E)/dT (units Joules/sec)
(for monoenergetic beams)
Define particle fluence:
This is a measure of the particle………
dN number of particles incident of sphere with cross-sectional area dA:
phi=dN/dA (units M^2)
This is really a measure of the particle intensity, the intensity of the particle beam.
If we have 106 particles incident on a sphere of radius 1 cm, what is the particle fluence?
The cross-sectional area subtended by the sphere is π times the square of the radius of the sphere, or π cm2, so the particle fluence is 106 divided by π particles per square cm.
Define energy fluence:
relax psy to phi
for monoenergetic beams
Radiant energy (RE) (=dN.E)/cross sectional area of sphere (dA)
Psy = dE/dA = phi*E (units joules/M^2)
phi is particle fluence
(i.e phi=dN/dA (units M^2) where dN is number of particles with energy E)
To relate fluence (particle or energy) to a polyenergetic beam it is necessary to:
Use an energy spectrum. Ie. a energies within a range [E,E+dE]
Therefore count the number of incident particles with energy within the range [E,E+dE] incident of sphere with cross-sectional area dA (i.e. the combined mono energetic particle fluences):
combined mono energetic particle fluencies =dPsi
phiE = dPsi/dE
psyE = E*(psiE)
Fluence rate can be described in terms of?
Units for each?
Particle Rate (just called fluency rate psi w/dot on top) or Intensity
Fluence (particle) Rate = dPsi/dt, Fluence rate is the fluence per unit time, or the number per unit area per unit time. So more particles going through this area per unit time, the more intense the beam than if fewer particles per unit time.
(units m^2/second)
Intensity (Energy Fluence Rate) = Energy Fluence/second = (dPsi*E)/dt = dPsy/dt
(units J/m per second)
The energy of photons is imparted in a 2-step process:
How does this relate to the concepts of Kerma and dose?
The energy of photons is imparted in 2-step process:
○ First step is energy TRANSFERRED to charged particles
○ Second step is energy ABSORBED by secondary charged particles
● Kerma refers to energy transferred to a volume: Taking into account the energy transferred from (photons) to charge particles within a volume:
K = (Rin)un - R(out)un
● Dose refers to energy imparted to a volume: Takes into acount energy transferred from photons and charged particles to charged particles within a volume.
D = (Rin)un - R(out)un + (Rin)ch - R(out)ch
● For charged particles energy loss is directly ……..
● Energy transfer is the energy imparted to a …….. ……….
● Energy absorbed is the ………. loss experienced by a charged particle, described by the …….. stopping power.
○ Defined as the energy ……….. per unit …….
● Energy deposited is the energy …….. or …….. in a single …….
● Energy imparted is the sum of … …………. … …….. minus … …………. … ……..
● For charged particles energy loss is directly absorbed
● Energy transfer is the energy imparted to a secondary electron by a photon.
● Energy absorbed is the collisional loss experienced by a charged particle, described by the collisional stopping power.
○ Defined as the energy absorbed per unit mass.
● Energy deposited is the energy transferred or absorbed in a single interaction
● Energy imparted is the sum of all energies (charged and uncharged particles) entering the volume of interest minus the energies leaving the volume.
What is the restricted collisional stopping power?
What fundamental metric does it contribute to?
The Restricted Linear Collision Stopping Power is the energy lost by a charged particle through hard and soft collisions over a unit path length, minus the total kinetic energy of delta rays with Ek over a set limit.
This is the amount of energy absorbed by a material.
If scaled by the Energy fluence will give DOSE - I.e the energy ABSORBED
Define KERMA
How does it relate to fluence?
Kinetic Energy Released per unit mass
K=(d._Etr)/d.mass
It is the energy transferred to a mass.
For mono-energetic beams:
K = ENERGY fluence(Utr/density) i.e.
Fluence x mass linear energy Tf coefficient,
Where the LET coeff Utr = Etr/hv (Etr=Eabs + E rad)
Units = J/Kg (Gy)
Difference between Gy and sievert:
1 Gy is the deposit of a joule of radiation energy per kg of matter or tissue. 1 Sv = 1 joule/kilogram – a biological effect.
Define collisional KERMA:
Like restricted LET, Kcol ignores average energy lost through radiative processes from excited electrons (Bremsstrahlung or annihilation).
Kcol = KERMA(1-_g)
where _g is the Average fraction of energy lost through radiative process
Units J/Kg
Another way to think about it:
Ktot = (Rin)uncharged - (Rout)uncharged
Kcal = (Rin)uncharged - (Rout)uncharged - Rradcharged
Rradcharged = Radiative losses from charged particles released in the volume (V). THIS is Regardless of whether radiative loss happens outside of V, so long as the secondary e came from V.
When is Kcol approximately = KERMA?
When radiative losses from charged particles is negligible.
Ktot = (Rin)uncharged - (Rout)uncharged
Kcal = (Rin)uncharged - (Rout)uncharged - Rradcharged
Rradcharged = Radiative losses from charged particles released in the volume. (V). THIS is Regardless of whether radiative loss happens outside of V, so long as the secondary e came from V.
In Low Z most loss is collisional
For polyenergetic photons:
Collisional kerma is related to fluence by
KERMAcol is related to energy fluence by mass energy Tf coeff (Uen, which = Utr(1-_g)), such that:
KERMAcol = Uen*ENERGY FLUENCE
Similarly for polyenergetic photons,
but replace Uen with _Uen,
_Uen = the mass energy absorption coefficient averaged over the energy fluence spectrum
Why is collisional KERMA usually more applicable than Dose?
Since most bremsstrahlung photons escape from the volume of interest, collision kerma is more applicable
Compare dose to collisional KERMA
Kcol: indirectly ionising (uncharged particle) Energy Tf to electrons (charged) in a volume.
Dose: uncharged and charged (direct and indirect ionisation) Energy Tf to a volume.
Kcol = (Rin)uncharged - (Rout)uncharged - Rrad (Rrad = Bremsstrahlung released by e liberated within V)
D = ((Rin)un - (Rout)un) + ((Rin)ch - (Rout)ch)
I.e D considers charged particles leaving V.. ((Rout)ch)
When does D = Kcol (what is this called)?
Charged particle Equilibrium (CPE):
If Kcol considers uncharged E entering = (Rin)un, and uncharged E leaving = (Rout)un:
Kcol = (Rin)un - (Rout)un - Rrad
And D considers both charged and uncharged particles:
D = ((Rin)un - (Rout)un) + ((Rin)ch - (Rout)ch)
Then D= Kcol when energy in of charged particles ((Rin)ch) = (Rout)ch. Therefore, (Rin)ch - (Rout)ch = 0:
D = ((Rin)un - (Rout)un) + 0 = Kcol
Besides (Rin)ch) = (Rout)ch, for true CPE to exist KERMA is constant (in real life KERMA attenuates, but transient CPE does)
Why is it important to be able to relate Kcol to D
Using CPE, a quantity can be calculated (Kcol) from a quality that can be measured (Dose).
Under CPE:
D = Kcol
D = Energy Fluence*(Uen/density)
How would you graph the relationship between Kcol and Dose
X-axis - Depth in medium (arb units), show Dmax
Y-axis - Relative Energy per unit Mass!
Show:
D build up, start a bit above y=0 (i.e electrons from air), end build up at Zmax the graph D as linear decline.
Kerma linear decrease with depth intersects D at Dmax.
Mark this point as “CPE”, then cont Kcol just below D
Beta = D/Kcol, mark beta<1, beta=1
What is Equivalent Dose:
Units?
● Radiation-weighted dose quantity, taking into account the type of radiation that produces the dose. It is tissue independent.
● To account for different biologic effect of different radiation qualities.
● absorbed dose multiplied by a radiation weighting factor
● Unit is J/Kg or Sievert
What is Effective Dose:
Units?
Accounts for tissue(s) irradiated
● the summation of all tissue equivalent dose, each multiplied by a tissue weighting factor.
● Accounts for the relative difference in sensitivities of different tissues.
● Represents stochastic effects (eg. cancer and hereditary)
● Eg. difficult to produce hereditary effects from irradiation of hands and feet.
● Unit is Sievert
Outline how to get from dose (e.g to 2 tissues) to Effective dose
1) convert to equivalent dose by multiplication with corresponding radiation weighting factor. (this is based on whole body radiation)
2) Convert this product(s) (i.e dosexWr) to Effective dose by multiplication with relevant tissue weighting factor Wt
3) If multiple areas irradiated sum up the above products to arrive at Effective dose.
Define Exposure:
Radiation exposure is a measure of the ionization of air due to ionizing radiation from photons. Units C/Kg (or R)
dQ/dm
Where dQ is the sum of ion charges| produced by electrons or positrons created or liberated within a mass of air (dm) by photons, divided by that mass.
Exposure (X) can be related to Kcol and K (air) how?
If we know the amount of coulombs of charge created per unit of energy deposited (electron charge/Wair)
Then this number x the collisional Energy deposited (Kcol) will give exposure X = kcol*(e/Wair)
e=1.602 x 10^-19 C
Wair = 33.97 × 1.602 × 10^-19 J/ion pair:
e/Wair = 1/33.97 J/C
X=Kcol1/33.97
Ktot = “Air KERMA” = (X/(1-_g))(Wair/e)
= 33.97(Exposure/(1-g))
Superficial and orthovoltage refer to what range of energies
Photon kV
Superficial: 50-150 kV
Ortho: 150-500 kV
Superficial XR therapy units typically deliver 90% of surface dose to ….. mm deep.
Superficial XR therapy units usually delivers 90% of surface dose to 5mm deep
Orthovoltage XR therapy units typically deliver 90% of surface dose to ….. mm deep.
Orthovoltage XR therapy unit usually deliver 90% of surface dose to 2cm deep
For superficial and ortho voltage machines beam hardening is expressed as:
Define this.
Degree of hardening is expressed as the Half Value Layer
The thickness (in cm) of a specified material that when introduced into the path of the beam reduces the EXPOSURE rate by half.
Essential components of a kilovoltage treatment machine:
1) kV generator:
○ supplies low voltage to the filament, enabling thermionic emission
○ Also supplies voltage across the anode and cathode to accelerate electrons.
2) X-ray tube
3) Beam collimation (primary and secondary/interchangeable) and applicators
4) Support stand
5) Safety features: eg. interlocks, emergency stops.
Clinical requirements of a linear accelerator (actual dumbass exam question):
Clinical requirements of a linear accelerator: ○ Compact ○ Reliable ○ Safe ○ High accuracy and precision ○ Well defined and uniform beam ○ Stable
Outline the essential components of a kilovoltage treatment machine XR tube
Kv Tx machine X-ray tube:
○ cathode and anode separated by space within a vacuum.
○ electrons are accelerated from cathode (filament) to anode (target) producing x-rays
○ electron energy >200 kV can eject secondary electrons from the anode, and hooding prevents buildup of electrons in the wall, forming electrostatic charge.
The basic components of beam collimation and application in kV treatment machines:
Consideration of source to surface is needed why?
Beam collimation and applicators:
○ Primary collimator is a conical hole in a block of lead.
○ Interchangeable applicators are used for secondary collimation- different sizes and shapes are available
○ As SSD decreases, dose rate increases.
Essential components of a linear accelerator:
1) Power supply and pulse modulator: flat topped pulses of ~50 kV delivered to simultaneously to magnetron/klystron and electron gun.
2) Electron gun
3) Microwave power source (Magnetron or Klystron)
4) Accelerator Waveguide (Travelling or standing)
5) Bending magnet - typically 270deg
6) Machine Head:
■ Target
■ Primary collimator
■ Flattening filter
■ Scattering foil to scatter the pencil beam
■ Monitor chamber
■ Field defining light and range finder
■ Secondary collimators and MLCs
What does the bending magnet in a linac do?
1) Beam bent through 270 (or some magnets 120) deg
2) Focuses beam
3) Removes chromatic aberrations (hence achromatic magnet) - filter low and high energy electrons
Compare a magnetron to a klystron
1) Magnetron generates microwaves (high power oscillator), Klystron amplifies them (has lower power microwave input)
2) Both exploit electron deceleratio to produce radiofrequency photons.
3) Magnetrons produce lower Energy Rf waves
4) Magnetrons are smaller
5) Magnetrons are cheaper, but live half as long
What does the wave guide do?
What are the 2 types? Which is smaller?
1) Accelerates electrons using interaction with microwaves
2) Focusing magnets center the electrons
Either Travelling (larger) or standing waveguide (smaller and able to fit in treatment head)
1st 3 parts of linac machine head (assume photon output desired) and their role:
1) Target
2) Primary collimator
● Defines maximum field size
● Usually conical
● Tungsten
3) Flattening filter (as opposed to scatter foil)
● Usually steel
● Flattens pencil beam
● Attenuates and hardens beam.
1st 3 parts of linac machine head (assume electron output desired) and their role:
1) Primary collimator
● Defines maximum field size
● Usually conical
● Tungsten
2) Scattering foil to scatter the pencil beam (not too thick or end up with bremsstahlung photons
3) Monitor chamber
● monitor photon and electron beam output
● monitors transverse beam flatness
● Backscatter plate reduces backscattered radiation into the monitor chambers
Parts of linac machine head (both for photon and electron output):
1) Target
2) Primary collimator
3) Flattening filter
4) Scattering foil to scatter the pencil beam
5) Monitor chamber
* ** Fixed wedge may be placed after chamber
6) Field defining light and range finder
7) Secondary collimators and MLCs
Besides the primary linac collimator, what are the other collimators that may be employed?
1) Secondary Collimators (typically tungsten): upper and lower jaws, provide rectangular fields
2) MLCs
3) Cerrobend
How does the physical penumbra differ between cerobend and MLCs
The physical penumbra with MLC is larger than that of Cerrobend blocks.
In addition to functioning as a tertiary collimator. MLCs can also function as:
How do varian and Electra differ?
Can function as upper jaw, lower jaw
Electra replaces one set of secondary jaws with an MLC - Advantage is field is smaller closer to the source so more responsive IMRT. Also smaller treatment heads
Varian add the MLC as the third pair of Jaws. Advantage thinner MLCs + keep secondary jaws (allows leakage reduction between tertiary leave - tertiary leaves have les leakage)
Outline the design of MLCs
○ two facing banks of tungsten leaves (usually 40-60 each bank)
○ width at isocenter ~5-10mm.
○ individually motor controlled and independent
○ Collimates the beam to give an irregular outline.
\ie. can match field to tumour shape.
○ Tongue and groove design
- Intraleaf transmission <2%, interleaf transmission <3%
○ Leaf ends rounded (e.g. Varian) vs focused (Siemens). So that field edge penumbra constant at all field sizes.
For MLCs
Intraleaf transmission < ?
Interleaf transmission < ?
What is the tongue and groove effect?
Intraleaf transmission <2%,
interleaf transmission <3%
Tongue and groove effect: Under coding between leave during dynamic treatment.
2 Problems with the design of MLCs?
2 Issues:
○ Light field underestimates radiation field by up to 1.2 mm
○ Leaf end transmission ~40%
Focused collimators are:
This allows?
What do MLCs do?
Are moved along a circular arc to be orthogonal to the the beam. This means the beam is attenuated the same at all angles. Meaning a sharp field edge.
Unfocused collimators move horizontally, such that when the field is large, oblique beams at the field edge pass through less attenuating collimator material.
MLCs use rounded leaf so beams at the field edge path through the same amount of material regardless offend size.
Disadvantage of rounded MLC leaf ends
When closed completely less density where leafs meet implies increased transmission.
What are the 2 types of linac wedge?
Besides speed, what are some major benefits of the more modern approach?
Physical/variable and dynamic.
1) Less beam hardening with dynamic wedge
2) Scatter is minimised with dynamic wedges, as the beam is open for most of the treatment. If we were to use a universal wedge for a breast field the scatter to the contralateral breast would be about 1.5% higher than with the dynamic wedge relative to the central axis dose.
When might a wedge be used?
- Improve dose distribution where field edges overlap - preventing to prevent high dose areas
- Compensate for oblique body contour
- Sloping target volume
Watch is the Wedge Angle?
How would you draw it?
The Wedge Angle - is defined to be the angle through which an isodose curve is tilted at the central ray of a beam at the 10 cm depth.
x-axis: Distance to Central axis (cm).
Y axis: Depth (cm)
Draw: angled isodose curves at 5, 10, 15 cm. The one at 10cm draw a line parallel to x-axis to make a triangle between wedge line and drawn line. show the angle (e.g. 45deg)
Physical wedges come in 2-types?
What are physical wedges typically made out of
1) “Universal” - built into the machine head of Elekta machines
2) Individual - mounted in the machine head
Dense Z - steel or lead
What happens to the wedge isodose line with depth and why?
Angle of tilt decreases because of scatter
A competing source with linac for high-energy photons?
Why may this option be better for some health systems?
Where else do you see this system?
Cobalt-60 machine - less moving parts, can be cheaper to maintain, but typically less conformal (but consider gamma knife).
How is Cobalt-60 created?
Co-59 + n → Co-60
I.e C-59 is bombarded with neutrons
The cobalt-60 isotope has a half-life of:
What does it decay to?
The cobalt-60 isotope has a half-life of 5.3 years so the cobalt-60 needs to be replaced occasionally.
Stable isotope (ie. same number of nucleons) is Ni-60
Cobalt-60 emits?
Largely gamma ray emitter, emitting 1.17 and 1.33 MeV gamma rays with an activity of 44 TBq/g (about 1100 Ci/g).
But obviously some beta particles are also emitted (e.g will scatter and cause Bremmstahlung in the source housing).
Cobalt-60 Machines are comprised of:
1) Source (1-2cm) - The occupation of finite physical space implies a geometric penumbra (i.e. not a point source). Source sealed in stainless steel, then sealed in 2nd steel capsule.
2) Housing = Source head.
○ steel shell filled with lead
○ device for bringing the source in front of an opening Source is returned automatically to the ‘off’ position in the event of power failure.
3) Beam collimation and penumbra:
- Like secondary collimators in linac. Typically hinged/circular trajectory to reduce transmission through corner edges when field large (=bigger TRANSMISSION penumbra)
In gamma decay a radioactive nucleus first decays by the emission of:
The daughter nucleus that results is usually left in an excited state and it can:
In gamma decay a radioactive nucleus first decays by the emission of an α or β particle.
The daughter nucleus that results is usually left in an excited state and it can decay to a lower energy state by emitting a gamma ray photon.
Sources of contamination in a Co-60 machine?
1) Beta particles: emission of electrons, electron interactions w/housing producing Bremmstahlung.
2) Gamma ray interactions w/: source, capsule, housing, collimators causing scatter and beam heterogeneity.
Define penumbra:
Types?
The region at the edge of the beam where the dose rate changes rapidly as a function of distance from the beam axis.
3 Types:
1) Geometric
2) Transmission
3) Physical
Define Physical Penumbra:
Physical penumbra is the combination of:
geometric, transmission and electron scattering at the beam edges.
■ Defined as the lateral distance two specified isodose curves at a specified depth.
Define Geometric Penumbra:
What is it independent of
Geometric penumbra: due to finite source size.
■ Related to source size, source to skin distance, source to diaphragm distance.
■ not influenced by field size (SDD is constant with increase in field size). I.e TRANSMISSION penumbra catches this.
How would you draw the calculation of geometric penumbra at depth:
Source - with length S
Collimators - With Source to Diaphragm Distance (SDD)
Skin line - SSD interval drawn
Depth line - d interval from skin to depth
Beams contra and ipsilateral to define interval pd
How would you calculate geometric penumbra (pd)?
pd = s(SSD+d-SDD)/SDD
where,
SDD = Source to Diaphragm (end of collimator)
s= horizontal length of source
d = depth below SSD (SSD usually = skin)
gamma ray sources are usually isotropic and produce ? photon beams.
X ray targets are non-isotropic sources producing?
gamma ray sources are usually isotropic and produce monoenergetic photon beams, while X ray targets are non-isotropic sources producing heteroge- neous photon spectra.
Superficial/kV machines produce what type of beam?
Heterogeneous spectrum with superimposed characteristic energy peaks at discrete energies (55Kev for Tungsten)
Describe the key components (including filtration) of a kilovoltage XR machine photon spectrum:
Y axis - Relative intensity
X Axis - Photon Energy (kev) 0 to 200 in 50 Kev steps
- The unfiltered spectrum (inherent beam) is a straight line hitting the x axis at the voltage difference between cathode an anode (i.e peak electron kinetic energy).
- Average intensity is at 1/3 max energy
- Filter removes all hv<10Kev then x^2 rise to peak.
- Intensity of each curve (e.g 50 kev, 100Kev) increases with energy
With increasing energy direction of X-ray emission becomes more?
What happens below 100keV?
With increasing energy direction of X-ray emission becomes more forward
Below 100keV there is approx 50% backscatter.
Describe the key features the photon spectrum of linacs:
● Maximum photon energy is equal to the maximum incident electron energy.
● Intensity reaches a peak at ⅓ of the maximum, dropping off below peak energy due to attenuation of low energy photons by the target material
● X-rays emitted are forward directed, hence the use of transmission targets (where x-rays are obtained on the other side) in megavoltage linacs.
Photon beams can be described in terms of what 2 aspects?
Quantity = intensity
Quality = penetrability of the beam
Beam Quality can be described in 2 ways:
In what conditions is each used?
1) The half value layer (HVL) is the thickness of specified material required to attenuate the intensity of the beam to half its original value.
○ This is used to describe low energy beams (together with kVp)
○ High energy beams are described using peak energy
2) Effective energy is another way of describing beam quality
○ the energy of photons in a monoenergetic beam that are attenuated at the same rate as the reference heterogenous beam.
○ the energy of a monoenergetic beam with the same linear attenuation coefficient as the given beam.
For kV photon beams what is the role of filters?
● For kV beams filtration is used to absorb unwanted lower energy photons and characteristic X-rays
● It also hardens the beam
For kV photon beams any single filter will produce characteristic XRs - how are the removed?
Combination filters are used to filter out characteristic X-rays produced by individual filters.
○ Thoraeus filter Sn-Cu-Al
○ K absorption of Sn and Cu is 30 and 9 KeV.
○ Attenuates the beam without unacceptable decrease in beam intensity
○ It is arranged with highest atomic number facing the x-ray tube hence Sn/Cu/Al with aluminium facing the patient
Why isn’t additional filtering needed in linacs?
For MV photons the beam is hardened by inherent filtration and by passing through the flattening filter hence no additional filter is needed to improve beam quality.
For SXR/keV machines.
Increasing:
1) Cathode-Anode voltage causes:
2) Cathode-Anode current causes:
1) Voltage increases = increased kinetic energy of electrons = higher energy hv.
Therefore:
i. more forward directed
ii. Mean energy, max energy, effective energy will increase
iii. HVL will increase
2) Current increased = more particles.
- More fluence
- Therefore less treatment time
- No change to energy of hv produced therefore no change HVL, average photon direction ect.
The penumbra is formed by:
It degrades the :
The penumbra increases with:
Due to:
● Penumbra is formed by transmission through the collimator edges and electron scatter, which degrades the beam edge.
● The penumbra increases with increasing beam energy due to increased lateral range of scatter outside the beam edge.
Without saying what they mean, list the key terms (ratios, and factors) used in dose calculations:
(There are 7)
1) Percentage Depth Dose (PDD) (%)
2) Tissue Air Ratio (TAR)
3) Peak scatter factor (PSF) or at low E Backscatter factor
4) Scatter Air Ratio (SAR)
5) Tissue-Phantom Ratio (TPR)
6) Output Factor Ratio (Total Scatter Factor)
7) Off-axis Ratio
In beam measurement: What is PDD?
What does it vary with?
Variation in dose at depth along central axis, compared with dose at reference depth (usually Dmax).
PDD = Dose at d/DMax x100 (units %)
Varies w/beam intensity, field size, SSD
What is TAR?
What key variable does it not depend on?
The ratio of the dose at a given point (Dd) in phantom to the dose in free space at the same point.
Since the primary beam is attenuated with depth, the TAR for the primary beam is only a function of depth, not SSD
What is its main use of TAR now?
What variables does it depend on?
It was initially developed to facilitate rotational therapy with its changing SSD, and is now used for irregular fields as well as stationary isocentric techniques.
● TAR varies with energy, depth and field size.
● TAR and PDD are interrelated- TAR can be used to convert PDD from one SSD to another (ie independent of SSD).
What variables does TAR depend on?
TAR varies with energy, depth and field size.
What is Backscatter Factor (or peak scatter factor)?
When is it called BSF vs PSF?
The ratio of the dose at the depth of maximum dose (Dmax) within the phantom to the dose in free space at the same point. I.e the extra dose contributed by backscatter.
Referred to as BSF at low photon energies where zmax=0
What is BSF/PSF critically dependent on?
BSF increases with increasing field size (i.e can make dose to skin pretty high with low energy beams)
Beam intensity
How high can orthovoltage BSF get?
For MeV beams?
For orthovoltage beams, BSF can be up to 1.5, which translates to 50% increase in dose near the surface compared to in free space.
For megavoltage beams, BSF is much smaller. Above 8MV, BSF is almost =1.
What is Scatter Air Ratio?
SAR = TAR(finite field sz) – TAR(zero field size)
Defined as the scattered dose at a given point in the phantom to the dose in free space at that same point
What is Scatter Air Ratio used for?
What variables does it depend on?
Gives the contribution of scatter to the dose at a point.
Depends on:
Beam energy, field size, depth
Based on TAR - therefore independent of SSD
What is the Tissue Phantom Ratio?
What TMR
Dose at same Source Axis Distance but different depth compared with reference depth ).
When the reference depth is Dmax then Tissue Maximum Ratio (TMR):
TMR = Dd / DrefMax
When is TPR/TMR used?
What does it depend on?
TMR = Dd / DrefMax
Used in isocentric techniques but for higher energy photons
Independent of SSD but dependent on field size and beam energy.
What is the output factor (aka)?
Output Factor (aka Total Scatter Factor): Described the effect of field size
The dose rate (dose/MU) at a reference depth for a given field size divided by the dose rate at the same point and depth for the reference field. Typically both measured at zmax.
OF= Collimator scatter factor x phantom scatter factor
What is Collimator Scatter Factor?
What calculation is it important to?
Collimator scatter factor
○ ratio of output in air for a given field to that for a reference field.
○ Reference field usually 10x10 and CSF for that field size is 1.
Output Factor (aka Total Scatter Factor): Collimator scatter factor x phantom scatter factor
What is Phantom Scatter Factor (aka?)?
What calculation is it important to?
Phantom scatter factor (relative peak scatter factor):
○ Ratio of PSF for a given field size at a reference depth to the PSF at the same point and depth for a reference field of 10x10 cm2
Thus, PSF normalized to 1 at 10x10 cm field
OF=Collimator scatter factor x phantom scatter factor
Define Off-Axis Ratio?
What is used for?
● the ratio of dose at an off-axis point to the dose on the central beam at the same depth in a phantom.
● Used for calculation of dose at points away from the central axis
What is Beam Quality?
The spectrum of photon energies that make up the beam.
○ For megavoltage beams it is described as the maximum energy of photons within the beam
○ For kV beams max energy or HVL is used.
How is Beam Quality measured?
Beam Quality is measured by calculating the dose rate at a depth of 20 cm in water compared to 10 cm in water- the TPR20,10 using an ionization chamber.
● Higher TPR20,10 means more penetrating beam.
What variable describes Beam output?
Determined by monitor units. (Units MU)
What are Monitor Units?
MU- 1 Monitor unit is the amount of charge measured by a monitor chamber where 1 cGy has been delivered at a reference depth (see below), with specified field size and radiation energy.
1) Depth of maximum dose in a water-equivalent phantom whose surface is at the isocenter of the machine (i.e. usually at 100 cm from the source) with a field size at the surface of 10 cm × 10 cm.
2) Depth in the phantom with the surface of the phantom positioned so that the specified point is at the isocentre of the machine and the field size is 10 cm × 10 cm at the isocentre.
The calculation of MU differs for fixed … and … treatments
The calculation of MU differs for fixed SSD and SAD treatments
MUs for fixed SSD are affected by:
How is it different for fixed SAD?
■ Affected by:
● Dose
● SSD
● OF (Output factor, aka Total scatter Factor)
● PDD (%DD)
● WF (wedge)
● CF (calibration factor- only if 1 MU is not equal to 1 cGy)
SAD uses Tissue Phantom Ratio
There dependent on TPR instead of SSD (PDD which relies on SDD also not needed)
Therefore: Dose, TPR, WF, OF, CF
Formula for MU fixed SAD
MU = Dose (cGy)/CF.(TPR.OutputFactor.WedgeFactor)
calibration factor- only if 1 MU is not equal to 1 cGy
Formula for MU fixed SSD
MU = Dose (cGy)/CF.(PDD.OutputFactor.WedgeFactor)
calibration factor- only if 1 MU is not equal to 1 cGy
IAEA states clinical radiation beam output has reference dosimetry determined by:
Dose in water under specific reference conditions: distance and depth,
field size,
material and dimensions of phantom,
ambient temperature, pressure and relative humidity
In terms of reference dosimetry, beam output is typical statred as:
Units for output for:
kV macines
Linacs
Basic output is usually stated as dose rate at point P at reference depth in water, for nominal SSD and field size.
● Output for kV machines is given in Gy/min, for linacs Gy/MU.
2 broad categories of dosimeter
Absolute - produces a signal in which a dose can be determined without calibration.
Relative - Relative dosimeters require calibration in a known radiation field
Examples of relative dosimeters:
○ standard free ionization chambers
○ cavity ionization chambers
○ phantom embedded extrapolation chambers
Examples of absolute dosimeters:
calorimetry, Ficke dosimetry, ionization chamber dosimetry (absolute and relative)
Units for Exposure
Unit is coulomb per kg (C/kg),
previous unit is the roentgen R (1R- 2.58 x 10-4 C/kg)
The average energy expended in air per ion pair formed (or the average energy required to create an ion-pair in air):
Wair
Where Wair= Ek/N
N is the mean number of ion pairs formed when initial kinetic energy Ek of a charged particle completely dissipated in air.
e=1.602 x 10^-19 C
Wair = 33.97 × 1.602 × 10^-19 J/ion pair:
e/Wair = 1/33.97 J/C
How can Exposure be related to dose?
Exposure X = dQ/dm
Kcol = Energy fluence x Uen
If Wair/e is the average energy deposited to create an ion pair with unit charge and there is X amount of charge, then the (Kcol)air per mass is:
(Kcol)air = X(Wair/e) or
X = (Kcol)air(e/Wair)
Since at CPE Dose = Kcol
Dose = X(Wair/e) @CPE
Conversion of dose to air to dose to medium is based on?
The two versions are:
Conversion of dose to air to dose to medium is based on cavity theory.
● The Bragg-Gray cavity theory: does not take into account the creation of secondary electrons within the dosimeter.
● The Spencer-Attix theory: accounts for delta electrons
Cavity theory relates the absorbed dose in the …….. ………… medium to the absorbed dose in the ……… medium.
Cavity theory relates the absorbed dose in the dosimeter’s sensitive medium to the absorbed dose in the surrounding medium.
The ranges of secondary charged particles produced in the cavity medium determines the description of?
Cavity sizes are referred to as small, intermediate or large in comparison with the ranges of secondary charged particles produced in the cavity medium.
The two key conditions for Bragg-Gray cavity theory
1) Cavity must be small so that its presence does not perturb the fluence of charged particles in the medium
● This condition is only valid in regions of CPE or TCPE
● There is always perturbation requiring fluence perturbation correction
● in practice only fulfilled by gas filled cavities eg. ion chambers.
2) The absorbed dose in the cavity is deposited solely by charged particles crossing it
● photon interactions are ignored
● all electrons depositing dose inside the cavity are produced outside
● no secondary electrons are produced inside the cavity and no electrons stop within the cavity. (this assumption ius different in Spencer-Attix)
Assuming the conditions of Bragg-Gray cavity theory are met, how is dose to medium determined?
One component of this formula highlights the key difference with Spencer Attix cavity theory
Knowing the mass (unrestricted) collision stopping power (UMCSP) for the material (s/p ), and cavity (s/p)cav
And Dose to cavity Dcav
Dose = Dcav x (UMCSPmaterial/UMCSPcavity)*(pertubation corrections)
i.e the dose to material is the cavity measured dose scaled by the extra stopping power.
The ratio (UMCSPmaterial/UMCSPcavity) can be abbreviated Smed,cav
■ The use of UMCSP rules out production of secondary charged particles (delta electrons) in the cavity and medium
Spencer-Attix theory accounts for?
But only those that?
It does this by?
Spencer-Attix theory accounts for?
But only those that?
It does this by?
○ accounts for delta electrons.
○ Some delta electrons have sufficient energy to leave the cavity which reduces energy absorbed in the cavity.
○ Therefore threshold energy Δ = energy of an electron with range equal to the mean chord length of the cavity.
■ Those with energies lower than Δ are considered deposited within the cavity.
■ Secondary electrons with energies equal or more than Δ are assumed to be part of the electron spectrum
Brief general formula for Dose in medium using Spencer-Attix theory
To capture dose deposited by Delta rays whose energies are not sufficent to leave cavity,
Use Restricted Mass Collisional Stopping Power (RMCSP), restricted to losses less than threshold Δ.
Dmaterial = Dcavity(RMCSPmaterial/RMCSPcavity)*(pertubation corrections)
The ratio (RMCSPmaterial/RMCSPcavity) can be abbreviated Smed,cav
For a thin walled ionization chamber, the dose to medium is given by (i.e. the generalised form for both theories):
Dmed = X(Wgas/e)(Smed,cav) * perturbation corrections
X = Q/m X(Wgas/e) = (Kcol)air = Dair
So
Dmed = Dair scaled by the ratios of either unrestricted (Bragg-Gray) or restricted to a threshold (Attix-Spencer) stopping power between medium and Cavity (the ratio Smed,cav)
The perturbation corrections used in calculations of dose to medium correct for what
pfl, pdis, pwall, pcel are perturbation corrections:
for electron fluence,
displacement of the effective measurement point,
wall,
and central electrode.
IAEA give procedures to be followed when calibrating a clinical photon or electron beam.
1) In order to relate measured charge to dose at a point in medium what 2 theories may be used?
2) What 2 protocols are there for calibration.
1) Spence-Attix or Bragg-Gray
2)
○ Based on air kerma in air calibration coefficients
○ Based on absorbed dose to water calibration coefficients
Absorbed dose to water for a reference beam of quality Q0 (usually ?) is
Absorbed dose to water for a reference beam of quality Q0 (usually Co-60) is:
D(w,Qo) = M(Q0) x N(D,W,Q0)
where the measured signal MQ0 (units nC) is the fully corrected chamber reading under reference conditions in the standards laboratory. This must be corrected for air temp/pressure/humidity, chamber polarity and voltage.
N(D,W,Q0) (Gy/nC)is the calibration coefficient in terms of the absorbed dose to water of the chamber obtained from the standards laboratory.
In addition to N(D,W,Q0) the calibration coefficient obtained from the standards laboratory. The measured signal is also corrected for?
he measured signal M is influenced by quantities that are not measured (and must be corrected for) eg:
○ ambient air temperature, pressure and humidity
○ applied chamber polarity
○ applied chamber voltage.
○ hence Mq= M x kTP x kpol x ks
When an ionisation chamber is used in a beam of quality Q that differs from Q0, the absorbed dose to water is:
the standard calibrated signal:
D(w,Qo) = M(Q0) x N(D,W,Q0)
scaled by ratio k(Q,Q0)
● where k(Q,Q0) corrects for differences between the reference beam and user beam qualities.
Beam profiles
● Measured:
● Usually performed in:
● The penumbra region is best measured:
Beam profiles
● Measured at multiple points on a plane perpendicular to the central beam axis.
● Usually performed in a water phantom using cylindrical ionization chamber
● The penumbra region is best measured using silicon diodes which have a smaller detection area and are more accurate for rapid dose changes.
Isodose charts are constructed by:
To give a depiction of:
Constructed by combining depth dose curves with the beam profile at multiple depths.
Depiction of dose distribution in a 2D plane.
The 2 parts of a radiation beam:
Each is affected by:
Primary and scattered
Primary only reflects beam quality, with dose decreasing linearly with attenuation.
Scattered depends on quality (i.e more elctron scatter with high E photons), field size, collimation, material
Descriptors of dose distribution:
1) PDD: surface dose, Build-up region, Dmax, exit dose (i.e close to exit less backscatter = from in dose at that depth)
2) Beam Profile: dose on a line perpendicular to central beam axis at a certain depth. i.e how dose is altered away from the central beam axis.
3) Beam flatness: variation of dose relative to the central axis over the central 80% of the field size at 10cm depth in a plane perpendicular to the central axis
4) Beam Symmetry: dose at a pair of points located equidistant from the central axis
What is PDD measured in?
What are the processes underlying it?
Measured in water phantom
Driven by beam energy, field size, field shape, SSD
What are the components of the PDD curve?
1) Surface dose
■ typically low for MV machines.
■ due to scattered electrons
■ Skin sparing effect increases with increasing energy.
2) Buildup region
■ As the high-energy photon beams enters the patient, high speed electrons are ejected from the surface and subsequent layers.
■ These electrons deposit their energy until they’re stopped downstream
■ Electron fluence and absorbed dose increase with depth until they reach a maximum, but because photon fluence decreases with depth, dose eventually begins to decrease past a certain point.
3) Depth of maximum dose
4) Dose at depth
5) Exit dose: i.e backscatter/scatter contribution will drop before the exit, causing a drop in dose.
What happens to Dmax as field size increases?
Confusingly Dmax does the same thing at very small doses. Why?
Dmax slowly decreases as field size increases (increasing scatter). This effect is more pronounced with beam strengths >6MeV
At very small field sizes (<30mm) Dmax decreases as well due to phantom scatter effects.
Define beam profile:
The variation of dose occurring on a line perpendicular to the central beam axis at specified depth.
● Represents how dose changes at points away from the central axis.
Regions of a beam profile:
3 Parts:
1) Central region includes doses over 80% of the central beam axis.
2) The penumbra region is between the doses of 80-20% of the central beam axis and is where there is rapid dose fall off.
3) Umbra is the region outside the radiation field.
- Dose is generally low and results from transmitted radiation through collimators and machine head.
ALSO:
● High energy photon beams can have lateral horns.
○ This is due to the flattening filter effect
In a beam profile, the Physical penumbra is the sum of
Physical penumbra (sum of geometric, transmission and scatter) : ● geometric penumbra due to finite source size ● transmission penumbra through collimators ● scatter penumbra within the patient
Physical penumbra is dependent on:
Physical penumbra is dependent on beam energy, source size, SSD, SDD and depth.
2 measures of the central beam profile
1) Flatness - variation across central region (F)
F = 100* (Dmax-Dmin)/(Dmax+Dmin)
For linacs at 10cm depth F required to be <3%
2) Symmetry: ratio of dose at 2 point equidistant, but on opposite sides of central axis - should be <2%
In terms of PDD curves, increasing field size:
1) Slowly decreases Dmax (as does decreasing field below 3cm)
2) Decreases the drop off of dose with depth (i.e
3) Build up region narrower
4) Increased skin dose.
PDD generally refers to depths greater than:
PDD generally refers to depths greater than the depth of maximum dose
PDD decreases with increasing depth due to both:
PDD decreases with increasing depth due to the inverse square law and due to attenuation of the radiation beam
PDD ……….. with increasing radiation field size due to greater ………. ……..
PDD increases with increasing radiation field size due to greater primary and scattered photons from the irradiated medium.
How does PDD change with increasing SSD?
Why
PDD increases with increasing SSD because inverse square variations over a fixed distance interval are smaller at large total distance than small total distance (i.e mean less at greater distance, therefore less PDD drop off)
How does Dmax and build up region change with increasing SSD?
It doesn’t
What happens to the PDD curve with increasing beam energy?
With increasing energy ○ Surface dose decreases ○ Dmax becomes deeper ○ Buildup region becomes broader ○ Falloff post Dmax is slower
What happens to the PDD curve with increasing beam energy?
With increasing energy ○ Surface dose decreases ○ Dmax becomes deeper ○ Buildup region becomes broader ○ Falloff post Dmax is slower
What happens to the PDD curve with a physical wedge?
Beam hardened (?less backscatter) VERY minimal increase in PDD
Compton scattering is largely proportional to?
It is largely independent of?
Compton scattering is proportional electrons/gram
Largely independent of Z.
Most tissues have roughly similar electrons/gram, yet significantly different?
Why is this relevant to MeV therapy but not keV?
Most tissues have roughly similar electrons/gram, yet significantly different electrons/cm.
For MeV machines where Compton dominates (ie. on the interval 100keV-10MeV), mass attenuation is similar, but tissues differ in their electrons/cm. Either way this change in the number of electrons leads to changes in attenuation (absorption) with tissue heterogeneities (e.g lung)
For MV beams the amount of attenuation in depends on?
Because?
Tissue heterogeneities therefore cause?
Amount of attenuation depends on electron density (electrons/cm3). Because Compton scattering dominates in this range (until 10MeV).
○ Increased or decreased scattered electrons impact on dose distributions at the interfaces of different densities.
Increased scattered electrons which impact on dose distributions at the interfaces. What happens at?
High to low density inhomogeneity:
High to low density inhomogeneity:
● there is an increase in dose on the low density side due to increased number of electrons from the high density side
● there is a decrease in dose on the high density side due to decreased backscatter.
Increased scattered electrons which impact on dose distributions at the interfaces. What happens at?
Low to high density inhomogeneity:
Low to high density inhomogeneity:
● Low side has increased dose due to back scatter
Note how asymmetric this is compared to the high to low case.
kV beams attenuation is largely due to?
As such attenuation depends largely on?
What tissue is this particularly relevant to?
The photoelectric effect
Therefore largely dependent on Z
Bone is high Z - greater attenuation at orthovoltage.
Air cavities have what effect on irradiation:
1) Causes electronic disequilibrium = loss of dose at beam edges - widening penumbra.
2) Primary beam minimally attenuated
3) Increased dose to soft tissues beyond cavity due to lack of attenuation.
For MeV range, as a beam passes through a metal prothesis, what happens to dose on the other side?
High Z and increased density
Therefore more PE attenuation (minimal importance in MV range)
More Compton attenuation
Therefore more attenuation.
But
As beam passes from high to lower density, more scattered electrons arrive at tissue at junction, between prothesis and tissue.
Changing patient contour relative to the source leads to?
If parts of the patient are closer to the beam source, the beam will be attenuated more than distant parts.
● Leads to undesired changes in isodose curves at depth.
Two broad approaches to improve changes in isodose curves at depth due to changes in patient surface contour?
1) Corrections to flat beam data
2) Compensators - bolus, treatment head compensator or wedge.
For uneven patient surface contours (i.e changing SSD), what corrections exist?
What data are they applied to?
When can they be applied?
Applied to flat beam data.
Can be used for angles of incidence up to 45o for MV beams and up to 30o for orthovoltage X-rays
1) Effective SSD method
2) TAR method
3) Isodose shift method
Name the 3 correction methods applied to flatbeam data to correct for irregular patient surface contours:
1) Effective SSD method
2) TAR method
3) Isodose shift m
The Effective SSD correction method for irregular patient surface contour:
adjust dose based on inverse square law and alteration of SSD.
The TAR correction method for irregular patient surface contour
Correction reflects ratio of the TAR dose ratio at a point compared with TAR dose ratio for flat surface data
The Isodose Shift correction method for irregular patient surface contour
Corrects the entire isodose chart
New curves = surface contour multiplied by a constant k (k is based on beam energy)
What is bolus?
Tissue density equivalent material applied to the surface of the patient to even out the surface contour, and intentionally bring Dmax closer to surface.
Increasing skin dose may be undesirable.
For electrons, bolus can be used as a build up material to increase skin dose.
Tissue inhomogeneity is now corrected by?
What were the previous methods (all based on what calculation)?
● Modern planning systems use dose kernels to adjust for dose inhomogeneities.
Previous methods used TAR (i.e independent of SSD):
● TAR method:
○ converts entire phantom to water equivalent, and calculates water equivalent depth of the inhomogeneity.
○ Does not take into account distance of inhomogeneity from point of interest
● Power Law TAR method
○ takes into account electron density of the inhomogeneity but only accounts for compton scattering
● Equivalent TAR method:
○ considers effects of inhomogeneity on primary and scattered radiation
Average mass stopping power is ……. for soft tissue than bone due to?
Average mass stopping power is greater for soft tissue than bone due to higher electrons per gram.
When a field is collimated asymmetrically (i.e regular field but not symmetric about central axis), what changes must be taken into account?
When a field is collimated asymmetrically, changes in collimator scatter, phantom scatter and off-axis beam quality must be taken into account.
When a field is collimated asymmetrically (i.e regular field but not symmetric about central axis) how can scatter be taken into account?
These assumptions are valid if:
By approximating with scatter data from symmetric collimators and phantom
These assumptions are valid provided the point of calculation is located away from the field edges to avoid penumbral effects.
● However, the primary dose distribution (and hence PDD, TPR) varies with distance from the central axis.
For irregular fields (i.e. other than square, rectangular, circular): TPR, TMR, Phantom scatter, collimator scatter can be approximated by what 2 approaches?
1) Effective field method = approximate rectangles, and collimator field
2) Clarkson integrated technique: Divides field into 5-10 deg sectors. Manually very time consuming.
Outline the basic approach of the effective field technique:
Approximate rectangles may be drawn containing the point of calculation to include most of the irradiated area surrounding the point and excluding remote areas.
● The rectangles are termed the effective field, while the unblocked, open field is termed the collimator field.
● Collimator scatter is determined by the collimator field whereas TPR, TMR and phantom scatter is determined by the effective field.
Advantages and disadvantages of MLC fields:
● Advantages:
○ motorized, rapid change in field shape
○ does not require manual alteration
○ reduced treatment time
● Disadvantages:
○ field outline not as smooth as custom cerrobend
○ interleaf and intraleaf transmission
Despite corrections for irregular or sloping surfaces in certain situations surface irregularity gives rise to unacceptable dose non-uniformity, or excessive irradiation of sensitive structures.
Methods have been devised to overcome this problem:
1) Wedges: Physical and dynamic
2) Compensators
3) Bolus
3. 1) Build-up for electron beams
The aim of shielding is to:
The aim of shielding is to:
Reduce transmission of the primary beam to <5%
The thickness of shielding depends on:
In general thickness > would give <5% transmission.
The thickness of shielding depends on:
Beam quality.
In general thickness >4.3 half value layers would give <5% transmission.
If you were going to make table to compare Fixed and SSD technique what you the headings be?
1) Patient setup: Nominal SSD vs target at machine isocentre
2) Positioning
3) Dose calculations
4) Uses/benefit
Fixed SSD treatment ● Patient is set up at: ● Requires patient positioning between: ● Uses .... to calculate depth dose curves ● KV machines use ... treatments
Fixed SSD treatment
● Patient is set up at nominal distance between source and skin surface.
● Requires patient positioning between different beam angles.
● Uses PDD to calculate depth dose curves
● KV machines use SSD treatments
Isocentric technique
● Patient is set up with target at?
● The SSD changes with each beam angle depending on the:
Isocentric technique
● Patient is set up with target at machine isocenter.
● The SSD changes with each beam angle depending on the depth of the target within the patient
● Dose calculations are done with TARs and TPRs to eliminate SSD dependence.
● Easy to treat multiple fields
Isocentric technique
● Dose calculations are done with ….. and ….. to eliminate … dependence.
● Easy to treat:
● Dose calculations are done with TARs and TPRs to eliminate SSD dependence.
● Easy to treat multiple fields
Simple photon treatment techniques are:
1) Single beam
2) Parallel Opposed
3) Multiple fields
Complex photon treatment techniques are:
1) Rotational (ARC therapy) special case of SAD technique. Largely supperceeded by IMAT.
2) 3D conformal radiotherapy
3) IMRT
4) IMAT = IMRT+ Arc therapy
Rotational therapy:
Has largely been superseded by:
Is a special case of:
IMAT
Special case of SAD - ie. beam rotates about continuously around the patient. - Little advantage over Tx with multiple stationary beams.
Describe 3D Conformal Radiotherapy, including the process of designing it.
1) Based on: 3D anatomical information and dose distributions that conform to the target volume.
2) Planning:
i. Acquisition (CT>MR>Fusion) = 2-3mm transverse slices digitally reconstructed into 3D
ii. Target volumes delineated on each slice. Including PTV to include consideration of system accuracy
iii. 3D planning software used to design fields and beam arrangement.
- BEV usually used to set margin
- Must take into account penumbra
iv. Optimisation
Optimising a 3D Conformal plan requires consideration of:
How does this differ from IMRT/IMAT/VMAT?
Beam angles, number of fields, weights and intensity modifiers.
IMRT ect plans are optimised using methods that are analytic (Backpropagtion), iterative, or both
What is IMRT
2 forms of modulation?
Intensity Modulated Radiation Therapy.
Uses non-uniform fluence (modulated by MLCs) to deliver a composite plan.
MLC modulation can either be :
Step-and-Shoot (Leaves shift between radiation deliveries)
Dynamic: Leaves move across field while radiation is being delivered.
Main benefit and main downside of Dynamic IMRT:
Dynamic IMRT reduces treatment time but requires additional QA to ensure accuracy.
Requirements for IMRT:
Requires
1) A treatment planning system capable of calculating nonuniform fluence maps and
2) A system of delivering the nonuniform fluence as planned.
IMRT Planning is described as?
What is involved:
Involves:
● “Reverse planning”: Target volumes and organs at risk are specified and a computer calculates the most appropriate field and fluence arrangement.
● Optimised: analytic methods (backprojection) or iterative methods (gradual adjustment of beamlets) or a combination of both.
What is IMAT
Major benefit and Major Pitfall
Intensity Modulated Arc Therapy:
Combination of arc therapy and IMRT, where gantry rotates around the patient with beam on, and MLCs shift dynamically during treatment.
○ This has the benefit of rapid delivery
but
Increases the volume of tissue receiving radiation.
What is VMAT
Volumetric-modulated arc therapy (VMAT):
A cone beam continuously rotates around patient. Each rotation = an arc and one or more arcs might be used. During each rotation, the cone beam is continuously shaped by the MLC. In addition, the dose rate and gantry speed are optimized to generate highly conformal dose distributions.
Major benefit and Major Pitfall of VMAT compared with IMRT:
VMAT and IMRT are equally effective for normal tissue sparing. However, the treatment time for VMAT is significantly shorter, thus benefits patients who require longer (30 minutes or more) treatment time.
Basic elements of QA/Commissioning of a machine intended to deliver IMAT
1) Physical testing of MLCs: Speed, acceleration, accuracy of leaf position sensors. General mechanical check.
2) Dosimetry: Including tier-leaf transmission, head scatter
3) Treatment verification: Phantoms + Ionisation chambers. Compare dose predicted by plan and dose distribution within patient
4) Regular QA.
Verify dose prior to start of treatment,
Daily check of dose to a test point in each field,
Weekly check of dose distributions cased on gantry and collimator positions
All machine elements checked yearly
Why are high Z materials chosen for the scattering foil?
High Z allows for scattering (Scatter power is proportional to Z^2), thin to prevent too much Bremsstrahlung.
How does collimation of electron beams differ from photon beam collimation?
● Electrons are not collimated by secondary or tertiary collimators as lateral scatter will increase geometrical penumbra
● A cone is used with multiple collimators to reduce lateral scattering and extends quite close to the patient surface.
● For further collimation, a cutout made of cerrobend could be attached the the end of the cone
4 fates of an electron passing through matter
1) Transmission
2) Elastic collision = scatter
3) In elastic interaction with
i. Electron - producing ionisation and excitation
ii. Nuclei - Bremsstallung (Radiative loss)
Typical energy loss/cm of a clinical electron beam passing through water:
2MeV/cm in H20
The rate of energy loss for an electron beam depends on?
1) Electron density of the material
2) Beam intensity
How does the atomic number of a material influence its electron stopping power?
Low Z materials have higher electrons per gram, these are less tightly bound.
Therefore mass stopping power is higher for low Z materials
Units for stopping power:
For mass stopping power
Mev/cm
(MeV/cm)/(g/cm3) = MeV.cm2.g^-1
As an electron beam passes through a medium mean energy ……. and angular spread ……
As an electron beam passes through a medium mean energy decreases and angular spread increases.
Scatter power is related to atomic number and Energy how?
Proportional to Z^2
Inversely proportional to E
Which are more likely to interact with nuclei, electrons or protons?
Electrons
Compare electrons to protons in terms of: Size Bremsstrahlung Scattering Energy loss per interaction Bragg Peak
1) Electrons are 1800 times smaller than protons
2) Electrons are more likely to interact with nuclei, creating Bremsstrahlung rads
3) Electrons are scattered through a wider angle
4) Electrons loose more energy per interaction
5) Multiple changes in direction smears out the Bragg peak for electrons
Electron beams begin as essentially mono energetic, however after passing through a scattering foil, monitor chamber, and air its spectrum broadens. Therefore beams are usually characterised at:
What other key descriptor is taken from the spectrum?
Electron beams begin as essentially mono energetic, however after passing through a scattering foil, monitor chamber, and air its spectrum broadens. Therefore beams are usually characterised at the body surface.
The most probable energy is the spectrum peak
What quick formula gives the mean energy at the surface (E), if the depth dose curve is known
Mean energy at the surface (E) = 2.33 x R50
Where R50 = E/2.33
3 instruments/techniques to determine depth dose distribution - give 1-2 key pitfalls of each.
1) Ionisation chambers (parallel plate or cylindrical), require correction for water, air stopping power ratios + lateral scatter in air is less which in turn decreases fluence.
2) Silicon diodes: Sensitivity may change over time, high temp and directional dependence
3) Film - Rapid and indeed of energy, however cannot be used in water phantoms
IAEA TRS 398 gives what
Measurement protocols
IAEA TRS 398
○ Beam quality for electron beam dosimetry is specified by R50 in cm, at 100 cm SSD, 20x20 cm2 for all energies in the clinical range
How does SSD calculation differ for electron beams compared with photons?
Often effective SSD is used = distance from effective source (virtual source) to nominal point of SSD (the edge of the electron cone applicator). If the applicator doesn’t touch the skin and air gap g exists. Dmax(g) refers to the Dmax when g exists (as opposed to Dmax(g=0))
For electron beams in water phantom, define (and include units): ● Rmax ● R90 and R50 ● Rq ● Range increases wit?
Units = Depth in water (cm)
● Rmax is the depth at which extrapolation of the PDD tail meets the bremsstrahlung background.\Practical range Rp is the depth at which the tangent through the steepest section of the PDD curve intersects with the extrapolation line of the bremsstrahlung background.
● R90 and R50 is the depth beyond zmax where the dose is 90% and 50%.
● Rq is the depth where the tangent through the dose inflection point intersects the maximum dose level.
● Range increases with increasing energy
For electron beam range in water, a simple rule of thumb
for R90, R80, R50
Conversely if you measure dose at R50, how can you estimate E
R90 = E/4 in cm (3.2 in modern linacs) R80 = E/3 R50 = E/2.33
R=R50*2.33
Practical range Rp is defined as:
Practical range Rp is defined as the depth at which the tangent plotted through the
steepest section of the electron depth dose curve intersects with the extrapolation line of the bremsstrahlung tail.
For field sizes greater than Rp lateral scatter equilibrium exists
Compared with MV photons, surface dose for MeV electron beams is relatively …….
Typically between % and %
in contrast MV photon beams are of the order:
% to %.
Surface dose for megavoltage electron beams is relatively large (typically between 75 % and 95 %)
In contrast to the surface dose for megavoltage photon beams which is of the order of 10 % to 25 %.
For electron beams, why is percent surface dose so high compared with photons?
Why does it increase with increasing E.
Surface dose high because:
- Directly ionising (i.e no indirect part)
- Low energy electrons are scattered laterally causing rapid build up of dose.
Dose beyond zmax, especially at relatively low energy, megavoltage electron beam PDD curves drop off sharply as a result of both:
Dose beyond zmax, especially at relatively low megavoltage electron beam PDD curves drop off sharply as a result of:
the scattering
and continuous energy loss by the incident electrons.:
For electron beam PDD curves, steepness of dose falloff becomes less with increasing energy due to?
Steepness of dose falloff becomes less with increasing energy due to increased lateral scattering.
For electron beams, for field sizes larger than Rp of the beam, what form of equilibrium exists? Therefore?
What does this imply for smaller field sizes:
For field sizes larger than Rp of the beam, lateral scatter equilibrium exists. Therefore past this point PDD curves are independent of field size.
For fields smaller than this R90 shifts toward the surface with and the relative dose at the surface increases.
Decreasing field size below the practical range results in what changes in central axis PDD curve
Below 5cm
For fields smaller than this R90 shifts toward the surface with and the relative dose at the surface increases.
Decreasing field size below the practical range results in what changes in central axis PDD curve
Below 5cm
For fields smaller than this R90 shifts toward the surface with and the relative dose at the surface increases.
Below 5cm this will result in greater energy than initially proposed
For electron beams, how is PDD affected by SSD
The PDD is not significantly affected by SSD. With higher energies, D90 penetrates a few millimeters deeper at extended SSD.
What are the standard requirements for an electron beam profile
1) Flatness:
- Sharp edges: Distance from 90% to beam edge should not be > 10mm on major axes and 20mm diagonal
- Uniformity index
2) Symmetry: any symmetric pair of points on the cross beam profile shouldn’t by more than 2%
For electron beam isodose curves, describe the appearance of low value curves (20%) at the field edges. Explain why.
What happens with lower of higher energy beams?
low value curves budge due to increased lateral scatter with lower energies.
Higher energy beams, especially with smaller field sizes, tend to constrict laterally, as such larger field sizes may be considered when using higher energy beams.
For electron beams, physical penumbra is defined as:
Physical penumbra is defined as 80% to 20% isodose lines, at a depth of R85/2.
For electron beams, how does SSD affect penumbra?
Penumbra increases with SSD
For electron beams, what method is used to correct for tissue inhomogeneities?
● The coefficient of equivalent thickness (CET) method is used to correct for inhomogeneities
● the CET is given by its electron density (electrons/mL) relative to water.
Attenuation by a material with thickness x is equivalent to attenuation of (x x CET) of water.
CET is given by:
E.g?
CET is given by its electron density (electrons/mL) relative to water.
● For dense cortical bone CET is 1.65, for spongy bone CET is assumed to be 1 (equivalent to water).
What happens to an electron beam passing through tissue then dense cortical bone?
Dense bone (CET 1.65) attenuates the beam more than tissue leading to reduced dose post bone. BUT backscattering increases the dose to tissue on the entry side.
In general, for electron beams, decreased scatter (e.g. due to small air cavity) results in?
In general, for electron beams, decreased scatter (e.g. due to small air cavity) results in loss of electronic equilibrium.
For small air cavities, electrons passing through will have minimal ……..and there will be loss of ……. ……..
○ This leads to hot spots where?
because
For small air cavities, electrons passing through will have minimal scatter and there will be loss of electronic equilibrium
○ This leads to hot spots behind the air cavity
Due to:
1) unscattered primary electrons plus
2) inwardly scattering adjacent electrons
With dense inhomogeneities electrons are scattered laterally due to?
○ Beyond the inhomogeneity, this leads to a?
and ? lateral to it.
With dense inhomogeneities electrons are scattered laterally due to greater mass scattering power.
○ This leads to a cold spot beyond the inhomogeneity and hot spots lateral to it.
For 6Mev and 12MeV electron beams, give:
1) Surface dose
2) Dmax
3) R50
For 6Mev and 12MeV electron beams: 1) Surface dose 6MeV = 73% 12Mev = 84% 2) Dmax 6MeV = 1.4 12Mev = 2.8 (minimal increase in DMax past this) 3) R50 6MeV = 2.4 12Mev = 4.8
I.e for increase from 6 to 12, DMax and R50 double then differences become small (especially Dmax)
Options for field shaping with electron beams:
● Electron applicators or cones.
● Lead or metal alloy cut-out for customized field shape.
● Shielding
● Blocking a portion of the electron beam field
Where are electron applicators or cones placed?
What materials are used for customized field shapes? What does the use of these require?
● Electron applicators or cones. 5 cm from skin surface.
● Lead or metal alloy cut-out for customized field shape. Requires simulation.
For electron beam shielding, how do predict the approximate lead thickness needed to reduce transmission to ?
Divide Rp by 10 for approx lead thickness (<5% transmission).
○ or (in millimeters) 0.5 x Ep +1
○ If shielding is insufficient, dose could be enhanced directly behind the shield.
Blocking a portion of the electron beam field produces changes in both:
○ If field is smaller than Rp dose to open portion is ……. due to ?
Blocking a portion of the electron beam field produces changes in the dose rate and distribution.
○ If field is smaller than Rp dose to open portion is reduced due to loss of electronic equilibrium.
What is usually necessary when using Internal shielding for electron beam therapy? Why?
Internal shielding can be used, usually with 1mm coating of wax to protect healthy surface from backscatter and lead toxicity.
Uses for bolus in electron beam therapy?
● For irregular surfaces, bolus can be used to flatten out the surface.
● Bolus is also used to increase surface dose and to reduce electron beam penetration
For electron beam therapy, typically a tumour would be covered by ?% isodose line?
Given this, how might you go about choosing an energy (Ep)?
Rule of thumb
Ep = 3.33 x R90 (in cm)
E.g the deep extent of a tumour is 4.8cm
Therefore
Ep = 3.33 x 4.8 = 15.8 (i.e. use a 16MeV beam)
For electron beam therapy, how would you choose an energy to avoid a critical structure at depth?
Rule of thumb
Ep = 2 Rp (in cm)
Compare how choice of beam energy effects DMax and PDD for:
SXR
MV photons
MV electrons
SXR: Dmax on skin
MV photons: Increased dose increases DMax and PDD
MV electrons: Nonlinear, machine dependent increases in DMax with E (i.e DMax changes little with E>12MeV). Drop off becomes less steep. And practical range increases.
Compare how choice of beam energy effects skin dose for:
SXR
MV photons
MV electrons
SXR: DMax on skin for whatever energy (50-250KeV)
MV photons: Increased skin sparing relative to DMax with increasing dose.
MV electrons: Decreased skin sparing with increasing E
Compare how choice of beam energy effects penumbra for:
SXR
MV photons
MV electrons
SXR: allow 0.5mm around PTV for any energy (i.e pretty negligible difference)
MV Pho: Penumbra widens with dose
MV e: With increasing E, Lateral contraction at high doses regions, BUT lateral bulging of low dose regions - which increases penumbra at depth.
What critical feature of the beam profile but be considered when choosing field size?
Penumbra must be accounted for when choosing field size.
How does field size effect PDD for:
SXR:
SXR: To a point Increasing field size Increases the area under the PDD -
e.g. for 100kV beam:
D90 for 2.5cm field is 2mm and D10 60mm (but this point is D20 for 10cm field)
D90 for 10cm field is 4mm
Junctioning divergent beams at the surface causes?
Due to?
Whereas separation at the surface could cause a ?
Junctioning divergent beams at the surface causes hotspots due to overlap at depth, whereas separation at the surface could cause a cold spot superficially.
To avoid hot/cold spots due to junctioning, what techniques could be employed?
○ Angling away of beams to match divergent edges
○ Beam separation for deep seated tumours
○ Half beam blocking
○ Feathered junctioning = moving of junction daily or weekly to avoid constant hot or cold spots at one point.
An example of orthogonal field junctioning:
Craniospinal irradiation
○ Bilateral opposed cranial field junctioned with posterior spinal field.
○ The cranial field is matched by rotating the collimator
○ The couch is also angled to match the divergent edge of the beam to the cranial border of the spinal field.
○ Or, the cranial field divergence is eliminated by half beam blocking
For electron-electron beam junctions, junctioning is usually done at?
The consequence?
Why is functioning beams of different energies more difficult?
● As most of tumours treated are superficial, junctioning is usually done on the surface.
● This will cause hot spots at depth
● Junctioning for beams with similar energies more straightforward
● Junctioning beams with different energies difficult due to lateral scatter at different depths.
Monitor units are derived from?
They are equal to?
What is the point of monitor units?
Measurement of ionisation within an ionisation chamber in the treatment head.
Equal to a specific dose of radiation at a specific depth, at a specific field size and SSD.
E.g 1 MU = 1 cGy at beam isoscentre, in 10cm depth in phantom,and 10x10 field size
Relate machine output to dose
Two different calculations (formula) used for monitor units exist, this is to describe?
Isocentric, versus non-isocentric fields
Dose calculation algorithms are used to account for?
can be divided into what groups?
Give examples
Used to account for varying surfaces, inhomogeneities, irregular fields, beam modifiers.
1) Correction-based algorithms (use correction factors to capture dose changes) :
Air gap corrections - e.g effective SSD, TAR/TPR method
Tissue heterogeneities - Ratio of TAR method
Irregular field - Clarkson segmental model
2) Model based algorithms
Convolution-superpostion
Monte-Carol (superior for inhomogeneities)
Roughly (very) outline convolution-superposition dose calculation.
What is it dependent on?
Captures attenuation of primary beam as it passes through voxels, kernel calculates scatter for each voxel.
Primary dose and scatter dose for each axel are combined to give dose distribution.
Dependent on image resolution and quality (e.g. motion blur).
Roughly (very) outline the Monte-carlo approach to dose calculation:
Pros and cons.
Simulates the transport of single photons within the phantom
● Scattering and attenuation of the photon and resulting electrons is taken into account based on attenuation data on each voxel
● The larger the number of simulated particles the more accurate the outcome
● Simulates thousands or millions of photons to generate a dose distribution
● Most accurate method but very time consuming and requires high end computers
Much superior to convolution-superposition for tissue inhomogeneities
Goal of immobilisation techniques
Goal is to reduce positioning accuracies:
1) During treatment (intrafraction) and
2) In between treatments (interfraction)
What are the requirements of an immobilisation device?
○ comfortable for the patient ○ reproducible ○ easy to setup and use ○ custom fitted ○ inexpensive ○ minimally affect dosimetry and not cause artifacts for planning
Types of equipment/techniques to ensure reproducibility of patient positioning:
Immobilisation devices
Setup checking
- Lasers
- Optical Distance Indicator
- Portal imaging
What is the purpose of portal imaging?
What are the modes of portal imaging available:
○ Allows comparison of daily treatment imaging to planning DRRs
○ Orthogonal films with MV or KV beams
○ Cone beam CT
What is image-guided radiotherapy (IGRT)
● RT that uses image guidance procedures for target localization before and during treatment.
● Used to identify and correct problems arising from inter- and intrafractional variations in patient setup, anatomy and organ and tumour movement.
Basic techniques of IGRT
Portal and radiographic images
On board KV devices - mounted 90 deg to gantry
CBCT
Fiducial markers are implanted within the imaging field of view as points of reference.
In terms of treatment quality control, define tolerance level:
Tolerance is a margin for continuation of treatment if errors are small and unlikely to lead to adverse outcomes
In terms of treatment reproducibility, patient contour may vary because?
How is this tracked?
● Patient contour may change between planning and treatment or during treatment
○ fluid accumulation
○ weight loss
○ immobilisation and setup errors
● SSD measured daily using the optical distance indicator and compared to previous measurements
● Tolerance levels dependent on treatment technique: palliative vs curative
In terms of accuracy of treatment delivery, key aspects of performance/accuracy can be broken into 2 groups:
1) Mechanical accuracy.
2) Radiation
In terms of accuracy of treatment delivery, define Mechanical Accuracy:
Mechanical accuracy = The performance of the hardware components of the treatment machine.
This includes jaw symmetry, light field concordance, the mechanical isocenter, MLCs and laser guides.
In terms of accuracy of treatment delivery, define Radiation Accuracy:
Radiation accuracy = The performance of the generated photon/electron beams in comparison to published data for that particular linac, including depth dose curves, beam profile, radiation isocenter and angulation of wedges.
Mechanical accuracy is monitored by:
Give tolerances for each thing.
Mechanical checks of:
● Light Field concordance (should match radiation field, including with collimation)
● Mechanical Isocenter (collimator 2mm tolerance, gantry 1mm tolerance). Uses centre finders.
● Lasers - lined up against each other and with gantry at 0, 180, 90 and 270 degrees.
● Optical Distance Indicator
Radiation accuracy is monitored by:
Radiation Checks:
● Radiation isocenter
circle.
● Beam energy
● Field flatness
● Wedges
○ Checked for amount of angulation
○ should be within 2 degrees of expected
How is the mechanical isocenter checked?
Rotation of the collimator, gantry and couch.
○ Collimator
■ Center finder is attached to accessory tray with pointer dipped in ink, resting on graph paper.
■ As collimator is rotated, displacement is marked and should be under 2mm.
○ Gantry isocenter
■ point of center finder touching point of horizontal rod
■ As gantry rotates through angles, relative position of rods are checked
■ displacement should be <1mm.
How is the beam isocenter checked for mechanical accuracy?
Radiation isocenter:
○ Point in space through which central axes of the beam coincide with rotation of the gantry and collimator
○ Using a thin slit of radiation
○ Film is irradiated through different collimator and gantry angles to create a ‘star’ pattern
○ lines should intersect within a 2 mm diameter circle.
How is beam energy monitored for a given linac?
What other aspect of the beam is checked?
Beam energy
○ PDD for beam energies measured and compared against published figures
○ difference should be <2%
Field flatness
○ should be fairly uniform across the central 80% of field width
○ can be measured in water phantom or radiochromic film.
○ should be between 90% and 103% of central axis dose.
What 2 types of error impact treatment accuracy?
● Systematic errors
○ constantly inconsistent error that is reproducible
○ inherent accuracy of treatment or positioning
○ eg. errors in patient setup, incorrect collimation, treatment plan transcription errors, incorrect calibration of measurement tools
● Random errors
○ errors due to unpredictable variations in measurements, fluctuate around a mean value.
○ Can be minimized with more precise measurements and improved patient immobilization
○ eg. patient movement, organ motion, inconsistent interpretation of skin marks and positioning.
Define systemic error:
Give relevant examples
Systematic errors
○ constantly inconsistent error that is reproducible
○ inherent accuracy of treatment or positioning
○ eg. errors in patient setup, incorrect collimation, treatment plan transcription errors, incorrect calibration of measurement tools
Methods/systems/equipment to avoid or detect dose delivery errors?
● Record and Verify System
● Select and Confirm
● Interlocks
What is the Record and Verify System (“R&V”)?
What does it include?
● Record and Verify System
○ Ensures that the planned treatment is delivered in a similar manner every day
○ Includes daily measurements of: ■ MU ■ beam energy ■ beam mode (photons/electrons) ■ jaw positions ■ collimator, gantry and couch angles ■ wedging ■ SSD
What is the Select and Confirm System?
What does it include?
○ Ensures correct treatment parameters
○ When a setting is selected, mechanical changes are checked to have occurred before treatment continues.
○ System also checks that the field correlates with the mechanical positions of the field, collimation and energy
○ Discrepancies are highlighted
What are interlocks?
What are some standard ones built into a treatment system?
Emergency shutoff mechanisms that are triggered by certain events
○ eg
■ last man out button
■ emergency stop button
■ positioning interlocks
● prevents beam on unless collimator, gantry and couch are in correct positions
■ beam interlocks
● prevents beam on unless jaws and MLCs are in the correct positions.
What is in vivo dosimetry?
What is its aim?
Measuring dose received by patient during treatment.
In contrast, in vitro dosimetry is measured in phantom
● Detect systematic errors that may have occurred during treatment planning
○ eg. dose in extremely non-uniform areas during total skin irradiation
○ or dose to pacemakers
Methods/systems to detect systemic error during treatment delivery:
1) Record and Verify Systems
2) Select and Confirm Systems
3) In vivo dosimetry
Types of dosimeters that may be used for In vivo dosimetry:
Advantages and disadvantages
● TLDs ○ small and thin ○ different sizes ○ do not require electrical wiring ○ disadvantage, requires time to read, can only be read once. ● Silicon diodes ○ Relatively small and capable of instantaneous readouts ○ requires wiring ○ requires calibration ○ sensitivity reduced with repeated exposures and requires re-calibration ● MOSFETs ○ small and free from wires ○ can be read more quickly than TLDs ○ relatively expensive ○ single use only
What is absolute dosimetry? Another name for it?
ABSORBED dose measurement, in ABSOLUTE TERMS, in clinical CONTEXT (Reference Depth and field).
AKA primary dosimetry
What is secondary dosimetry?
Dosimeter calimbrated against a primary standard
Universal requirements of a radiation measuring device
1) Precision = reproducible
2) Accuracy
Some general limitations to consider when choosing a type of dosimeter:
1) Linearity (ie. non-linearity at small/high doses)
2) Dose-rate dependency
3) Energy dependency
4) Directional dependence
5) Spatial resolution
6) Convenience
7) Penis
Love Does Sometimes Equal Direct Convenience
Broad categories of dosimeters:
1) Chamber
2) Film - Graphic vs cgromic
3) Luminescence
4) Semiconductor
5) Scintillator (plastic)
6) Chemical
Gieger counters - dont measure energy
Types of ionisation chamber:
Types of chamber dosiumeters:
1) Ionisation chamber
2) Thimble (Farmer) chamber
3) Parallel plate chamber
4) Extrapolation chamber
5) Brachytherapy chamber
How does an ionisation chamber work?
○ Gas filled cavity surrounded by conductive outer wall and central collecting electrode
○ Wall and central electrode separated by insulator
○ Voltage applied
○ When gas is ionized, electrons and positive ions are released. They are pulled apart towards the terminals, creating a current which is measured
Assuming this would ever be relevant to you (which it never will be). What would be a good use for Thimble (aka?) chamber
Aka Farmer chamber.
Can choose a specific volume.
○ Dose and depth dose for MV photons and electrons >10 MeV- large volume (0.6 cm3) for good signal
○ PDD and beam profiles for photon beams beyond zmax and electron beams (Parallel plate better for e-) - small volume (0.1 cm3) for good spatial resolution
Parallel plate chambers are characterised by?
When are they useful?
○ Used to measure surface doses and doses in buildup penis regions (thin electrode window and small electrode separation)
○ More accurate for electron PDD than thimble chambers
Parallel plate chambers with variable sensitive volume are called?
What are they used for?
● Extrapolation chambers
○ Parallel-plate chambers with variable sensitive volume
○ Used for measurement of surface doses and dosimetry of βenis rays and low energy x-rays
What are brachy sources measured/calibrated with?
Well chambers:
- Require sufficient volume for sufficient sensitivity (e.g for low dose rate) >250cm3
2 options for film dosimetry?
1) Radiographic film
2) Radiochromic film
What curve describes exposure vs Optical density?
The sigmoid Sensitometric Curve (or HD curve). The curve can be broken into initial tail (“Fog”), curve up (“toe”), linear portion and shoulder
What is radiographic film made out of?
What is the process
Thin plastic coated with silver bromide
Photoelectric interactions with silver bromide form LATENT IMAGE
Film is developed and fixed
Degree of blackening measured with DENSITOMITOR
In units of OPTICAL DENSITY(OD)
Net OD - recorded OD - the background density (“Fog”) of the film.
What are some uses of radiographic film dosimetry?
○ Good for relative dosimetry of electron beams- almost no energy dependence
○ For photons, silver (Z=45) preferentially absorbs radiation below 150 keV via the photoelectric process.
○ Mostly used for portal imaging and quality assurance eg. beam alignment, isocentric accuracy and beam profile.
What is radiochromic film?
○ Colourless film with nearly tissue equivalent composition
○ Develops blue colour upon radiation exposure (special dye is polymerized)
○ Self-developing, colour stability after 24 hours
○ High resolution
○ Much lower energy dependence (except for lower than 25 kV)
○ Insensitive to ambient conditions (although may be sensitive to humidity and UV light)
2 types of luminescence dosimetry:
1) Thermoluminescence (TLD)
○ Calcium fluoride or lithium fluoride
○ Release of trapped electrons in the form of light by heating (~3000C)
○ Plot of TL against temperature- ‘glow curve’
○ Area under the curve can be correlated to dose with appropriate calibration
○ Require calibration and annealing before usage
○ Used for in vivo dosimetry
2) Optically stimulated luminescence (OSL)
○ energy released by light from a laser
How does luminescence dosimetry work:
Luminescence Dosimetry:
Certain crystals when irradiated release free electrons which are trapped by impurities within the crystal. When these electrons are released by heating or exposure to light, UV, visible or infrared light is released.
Photomultiplier tubes detect the light emission and convert it into an electrical signal which is proportional to the dose received.
Types of semiconductor dosimetry?
Give uses and pitfalls of each.
1) Silicon Diode:
○ Diodes are smaller and more sensitive than ion chambers, and can be damaged by repeated radiation
○ Has angular and temperature dependence, requires calibration
○ Useful in electron beam dosimetry and small field sizes (where ion chamber use may be difficult).
2) MOSFET (metal-oxide semiconductor field effect transistor):
Excellent spatial resolution
○ Threshold voltage is a linear function of absorbed dose.
○ MOSFET dosimeters cover the entire energy range of photons and electrons.
○ Limited lifespan as threshold voltage is permanently changed - therefore expense.
How does a MOSFET work?
Ionizing radiation generates a charge within the oxide that is permanently trapped, causing a change in threshold voltage (the voltage when the conducting channel connects the source, allowing significant current across the transistor).
○ Threshold voltage is a linear function of absorbed dose.
How does Plastic Scintillator Dosimetry work?
● Scintillator converts ionizing radiation into photons
● Photons are amplified by the photomultiplier tube and measured
● Plastic scintillators are almost water equivalent
● Nearly energy dependent
● Small in size (1 mm3) yet sensitive
When are plastic scintillators used?
● Used where high spatial resolution is required, eg. high dose gradient regions, buildup regions, interface regions, small fields and brachytherapy
● Good reproducibility and long term stability (no significant radiation damage up to 10 kGy)
● Independent of dose rate, direction, temperature or pressure
Key benefits of plastic scintillators?
High spatial resolution that is not direction dependent.
● Good reproducibility and long term stability (no significant radiation damage up to 10 kGy)
● Independent of dose rate, direction, temperature or pressure
Types of chemical dosimetry?
What are the uses?
1) Alanine - pellets. Used in high dose dosimetry (>10 Gy). Tissue equivalent
2) Gels: near tissue equivalent, can be moulded into complex shapes which is useful for brachy/IMRT
i - Ficke Gel
ii - Polymer gel
What is in a Geiger counter
Consists of GM tube and processing and display electronics
What is a Geiger-Muller (GM) tube and how does it work?
GM tube- outer cathode (-ve) wall, inner anode (+ve) wire. Several hundred volts difference. Filled with inert gas.
● Ionizing radiation produces free electrons which migrate towards the anode.
● The Townsend avalanche results in massive numbers of electrons reaching the anode
● The counter displays counts per second
Limitations of Geiger counters?
Does not measure energy (just counts) or differentiate radiation type
○ Very long dead times (insensitive period during which any ionizing event is not measured as a count)- not suitable for high radiation rates (may show zero reading)
What are phantoms?
Phantoms are used in radiation dosimetry (clinically and preclinically) to investigate and measure the effects of DOSE on an organ or tissue.
Can be varied in size, shape, density to model dose delivered in tissue/organ.
What are the 3 most common types of phantom
1) Water
2) Slab
3) Anthropomorphic
Describe a water phantom?
60cm3 (i.e 40cm2 field +20cm margin)
○ Waterproof ionization chamber can be placed anywhere and at any angle
○ Homogenous and nearly tissue equivalent
What are some disadvantages of water phantoms
Large/heavy, filled with water and electrical circuits.
Close but not exactly tissue equivalent.
Describe a slab phantom:
○ Square blocks of varying thickness made of water equivalent solids (same effective atomic number, number electrons per gram and mass density- g/cm3)
○ ionization chambers can be placed in pre-drilled holes to measure dose rates or film may be placed between slabs to measure beam profile and isodose distributions
○ Easy to place, requiring minimal setup
Describe anthropomorphic phantoms
Human shaped phantom including internal inhomogeneities
○ Slabs of tissue equivalent material arranged in the axial plane.
○ Contain holes for placement of ionization chambers and films can be placed between slabs.
Define radioactivity:
● Spontaneous transformation of an unstable nucleus resulting in release of energetic particles from the atom.
● Can be in the form of particles, electromagnetic radiation or both
Activity describes the number disintegrations over time (rate)
Define activity (in terms of radioactivity):
Disintegrations per unit time = A
A = decay constant(λ)xN(number or atoms)
A = Aoe^-λt
What are the units for radioactivity?
SI unit is becquerel (Bq), one disintegration per second (dps)
○ More common unit is the curie (Ci): the activity of 1g of radium per second
■ 1 Ci = 3.7 x 1010 Bq
Besides number of disintegrations activity can also be described as:
1) Specific activity: Bq/g
2) Apparent activity: the activity of a theoretical unsealed source that would give the same EXPOSURE rate at 1m
For radioactive sources exposure is often related to another physical quantity? Which comes in 3 forms:
Air kerma = Exposure.(W/e)/(1-g)
1) Air Kerma Rate: the kerma rate (air kerma over time) calculated using the (μ/ρ) value for air. Unit is Gy/Hr
2) Air Kerma Rate constant: Relates kerma at a distance from the source, to the activity of the source per hour: AKRC = (Kair.d^2)/A unit Gy.m^2
3) Reference Air Kerma = Is Kair @ 1 m so units Gy/hr @ 1m.
For Brachy sources, what is the dose rate constant?
● Dose Rate Constant is the dose absorbed to water, at a distance of 1cm along the transverse axis of a source with air kerma strength of 1, in a water phantom.
In radiotherapy, what are the 3 types of half life?
1) Physical : Thalf = 0.693/lamda
2) Biological: I.e clearance
3) Effective: λe = λp + λb
More interms of radiation than gangster rap, what is Mean Life?
Average lifetime of an unstable nuclei before decay.
Tavg = 1/lamda
= 1.44 x Thalf
What is radioactive equilibrium?
What are some forms of equalibria that may occur?
When the rate of daughter decay is equal to its rate of production.
When parent half life is only a little longer than daughter this will be TRANSIENT equilibrium
When parent half- life is much longer there will be SECULAR equilibrium
The name for a particle composed of two protons and two neutrons?
The α particle
○ Helium nucleus
How many known elements exist?
How many occur naturally?
Which are radioactive?
● 118 known elements ● First 92 occur naturally ● Elements with lower Z are more stable ○ As number of particles in the nucleus increase, forces keeping them together become less effective hence higher chances of particle emission ○ Elements Z>82 (lead) are radioactive
What are the 3 main radioactive series?
3 main series:
○ Uranium
○ Actinium
○ Thorium
Radioactive nuclides with very high atomic numbers emit a ? composed of ?
Radioactive nuclides with very high atomic numbers emit a particle composed of two protons and two neutrons- the α particle
○ Helium nucleus
When an alpha particle is emitted what is the new nucleus what else is released
α particle = two protons and two neutrons
there for mass reduced by 4, atomic number reduced by 2.
Q is the total energy released called the disintegration energy
○ This appears as kinetic energy of the α particle and the product nucleus
○ α particles emitted have kinetic energies of 5-10 MeV
α particles emitted have kinetic energies of
α particles emitted have kinetic energies of 5-10 MeV
Exam typical example of radioactive decay?
radium to radon
What is beta decay, what is a sister process that may occur under the right conditions
Besides the particle,what else gets emitted?
Process of radioactive decay with the emission of a positive or negative electron from the nucleus.
if Neutron:proton ratio low then electron capture may happen - leading to characteristic XRs as orbitals refill
Negatron = electron + antineutrino Positron = Positron + neutrino
Key factor in determining whether beta minus or plus?
Neutron:proton ratio
i.e reduce the ratio to make more stable:
When n»p make more p by emitting a minus (electron)
- and an antineutrino + Q
When p»n make more p by loosing a plus
- and an neutrino + Q
If the n/p ratio is low Electron capture can happy
Key example of negatron emission (aka)?
Cobalt-60
Key example of positron emission (aka)?
Fluoride -18 (for PET scans)
Could be an Ex question
What is electron capture?
What else is, or may be produced?
● An alternative to positron emission for low n/p ratio nuclides.
● One of the orbital electrons (usually K-shell) is captured by the nucleus, transforming a proton into a neutron.
Results in characteristic XRs and possibility of Auger electron (less likely with Z>30)
Besides emission of alpha particles and beta decay what other activity occurs, and when?
An excited nucleus may transition into a stable state my emitting a photon = GAMMA DECAY
e.g after negation decay of Co-60 to Ni-60, further transition without change of atomic/mass numbers by emitting 2 gamma rays
INTERNAL CONVERSION - excess nuclear kinetic energy passed to orbital electron which is ejected.
Two processes that stabilise the energy of a nucleus without conversion of a proton or neutron?
Gamma decay - energy released as photon
Internal electron - energy transmitted to inner orbital electron
Rules for guesstimating properties of an electron PDD
1) Surface dose = 73+Energy
e. g 6MeV SD =79%
2) “4,3,2 divide rule”: Dmax = Dose/4, R90 = D/3, R10 = Dose/2.
E.g 6/4 = 1.5cm = Dmax, 6/3 = 2cm = D90, 6/2 =3cm = R10 (therefor D50 = 2.5cm)
