Physics Flashcards
cWhat does pair production produce and when does it occur?
A high energy photon disappears and produces an electron and positron.
To produce a particle E = mc2
Photon energy must be >1.022 MeV, the rest mass energy equivalent of the created electron-positron pair. This amount of energy is just sufficient to provide the rest mass of the electron and positron, 0.51 MeV each.
The nucleus recoils with negligible energy
Annihilation radiation
Electrons travels through matter undergoing collisions until brought to rest
Positron travels in the same way until, when nearly at rest
Annihilates with a few electron
Converts mass back to energy. . As the positron comes to rest, it interacts with an electron in an annihilation reaction and is
replaced by two photons, each having an energy of 0.51 MeV and moving in opposite directions.
When does probability of pair production increase?
Increases with increasing photon energy
Increases with increasing atomic number
Z squared
What is Compton scatter and when does it occur?
Incident photon interacts with a free or outer shell electron
A portion of incident energy of the photon is transferred to an electron in the form of kinetic energy - recoil electron
The incident photon (now called a scatter photon) is deflected in a new direction with less energy
Most common thing to happen at MeV energies in radiotherapy
What affects the energy of the recoil electron in Compton scatter?
Energy gained by recoil electron depends on :
Energy of incident photon Ey
-Higher photon energy -> more energy available to transfer
Angle of photon scatter 0
- Larger scattering angle of photon -> more energy transferred to electron
Note recoil electron can only go in direction +90- -90 degrees (no backscatter)
As energy increases scattering a forward direction more likely
The probabilty of Compton scatter increases with…
increasing electron density (hydrogen rich compounds_
The probability of Compton scatter decreases with
incident photon energy
Describe Rayleigh scatter and when it occurs?
Change in photon direction. Only occurs at low energies (<10KeV)
Probability of Rayleigh scatter increases with
Atomic number
Z squared
Probability of Rayleigh scatter decreases with…
increasing incident photon energy
Describe photonuclear interaction and when it occurs?
• A photon is absorbed by a nucleus, knocking out a nucleon. • Process is called photodisintegration. – Most common version is (γ,n) interaction – Neutron ejected from nucleus • Only at very high energies (and high Z) • Results in induced radioactivity in Linac
Probability of photonuclear interaction increases with…
atomic number, z and with incident photon energy
Describe photoelectric absorption and when it occurs?
A photon imparts all of its energy to an inner orbital electron
The photo vanishes and an electron may be ejected from atom producing an ion pair
Ejected electron will have energy of photon minus electron binding energy
Stage 2:
Space is left so electron has to fill space in energy shell which produces either characteristic x-rays or auger electrons
Occurs at low energies
Photoelectric absorption decreases with increasing..
Incident photon energy
Photoelectric absorption increases with
Atomic number, z cubed
What is the electron binding energy?
The energy required for an electron to escape the atom
What is the charge of a proton?
+1.6 x 10 ^-19
What is the mass of a proton
1.7 x 10^-27 (kg)
What is the charge of a neutron?
0
What is the mass of a neutron?
1.7 x 10^ -27
What is the mass of an electron?
9.1 x 10 ^ -31 (kg)
What is the charge of an electron?
-1.6 x 10^-19
What is the atomic number?
The number of protons in the nucleus
What is the atomic mass number?
The number of nucleons (protons and neutrons) in a nucleus
What is an isotope?
It has a same number of protons but a different number of neutrons
What is an element?
Kind of matter that cannot be decomposed into two or more simpler types of matter
Describe the names of the electron shells and how many electrons are in each one- starting with nearest to nucleus?
K 2 (greatest electron binding energy) L 8 M 18 N 32 O 32 P 32
What is excitation?
Electron receives sufficient energy to raise it to a higher energy level. Less than 10eV energy needed.
What is ionisation?
Electron receives sufficient energy to overcome the binding energy to escape the atom. In the order of 10eV
What is the equation for frequency and wavelength?
V (freq) = c / λ
c = speed of light 3 x 10^8m/s
λ = wavelength
Speed remains constant so wavelength and freq must vary together. High freq + short wavelength
Describe the electromagnetic spectrum of radiation from short wavelength and increasing?
Gamma rays 0.0001nm XRs 0.01nm-10nm UV Visible light 400-740nm Infrared 740nm - 0.7cm Radiowaves 1mm- 100km
What is the equation for the energy of a photon?
E= hc/λ
h= plancks constant 6.63 x 10^-34 c= speed of light 3x10 ^8m/s λ= frequency
What are the units of radioactivity?
Becquerels (MBq/GBq)- SI units
Curies
What is an alpha particle?
2 protons + 2 neutrons
Charge +2
Velocity 6% speed of light
What is a beta particle?
Either an electron or positron
What is the difference between an XR and a gamma ray?
XRs are created by accelerating electrons hitting a target
Gamma rays are emitted from atoms of unstable isotopes (unable to change the rate of production or energy)
XRs can have higher energy than gamma and vice versa
Describe the XR spectrum and what information it provides?
Range of photon energies are produced:
- continuous spectrum- results from Bremstrahlung and depends on energy of incoming electrons and on atomic number
- Discrete spikes of particular energy - particular to target material and are characteristic part of the XR spectra
Provides info on the quality of the beam (ability of beam to penetrate an object)
How do you generate an XR?
Electron source- eg tungsten wire heated by high electrical current -> thermionic emission (electron cloud)
Target -> high atomic number and melting point -> Bremstrahlung
Power supply -> generated positive charge on the target -> higher the charge, higher energy XRs produced
What is attenuation?
Loss of photons as beam penetrates some materials
= absorption + scatter
How do you calculate the intensity of a photon beam after it has travelled through matter?
I = Io x e^ -µx
Io = intensity beam on entering material µ= linear attenuation coefficient (m-1) x= thickness of the material
What is the linear attenuation coefficient and mass attenuation coefficient?
Linear attenuation coefficient (µ) = fraction of attenuated incident photons in a monoenergetic beam per unit thickness of material (m^-1)
Mass attenuation coefficient (µ/p) - divide the linear attenuation coefficient by the density of the absorber (p), units m^2kg^-1 (per unit mass rather than unit path length)
Both vary with energy of the beam
What is the k edge in photoelectric absorption?
The energy at which the photon energy equals binding energy of K (inner) electron shell.
the K-edge is a sudden increase in x-ray absorption occurring when the energy of the X-rays is just above the binding energy of the innermost electron shell of the atoms interacting with the photons. Results in sudden attenuation. Due to photoelectric effect
In water what is the most likely photon interaction?
Atomic number = 18
<25kv photoelectric
25kv - 25mv compton scatter
>20MV pair production
What is coloumb force proportional to ?
charge of one particle (q1) x charge of other particle (q2) / distance between their centres squared (r^2)
What are the three types of charged particle interaction?
Soft collision, hard collision and radiative losses
What is a soft collision and when does it occur?
Charge particle interaction
Occurs when impact parameter (b)»_space;a (atomic radius) - particle is not passing near the atom
Small amount of energy transferred to orbital electrons:
- excitation of atomic electron to a higher level which returns to ground state with emission of a photon
- ionisation of atom by excitation of a valence electron-> transfer a few eV energy to a medium
Accounts for half of energy loss but by far most common interaction just small amounts of energy lost
What is cerenkov radiation?
In a certain material highly energetic charged particles can travel faster than the speed of light- very small part of energy (<0.1%) of soft collisions is emitted as a coherent bluish white.
Describe hard collisions and when they occur
Charged particle interaction
Occur when impact parameter similar to atomic radius (b=a)
High speed electron knocks out an inner orbiting electron
Vacancy filled by either:
- electron dropping down emitting photon
- energy transferred to an outer electron emitting it from the atom -> auger electron
Possibly delta ray ejection- outgoing electron has sufficient energy to produce secondary ionisations
Describe radiative losses in charge particle interactions?
B«a>95%- elastic nuclear scatter interaction- electron deflected without losing energy
2-3% cases - charged particle passing near a nucleus is deflected with sudden deceleration due to couloumb force and loss of energy -> Bremstrahlung (braking radiation) -> energy lost emitted as photon
Main source of medical XR radiation
In low atomic number materials little energy lost in this way</a>
How do you quantify energy transfer?
Stopping power (property of material) - loss of energy/unit distance by charged particle Linear energy transfer (property of radiation) - gain of energy/unit distance of media from the charged particle
What is stopping power?
The rate at which energy is lost along a charged particle track.
Units = J m^-1
Usually shown by dE/dx
Describe the energy loss/stopping power of a charged particle?
Graph dE/dx vs distance of penetration
Low constant rate of energy loss immediately after entering a medium. Towards the end of the path, the rate of energy loss rises dramatically (Bragg peak) then falls to zero
What is the rate of energy loss of a charged particle with distance proportional to?
Proportional to square of a charged particle (i.e. an alpha particle (+2) loses energy 4x as fast as a proton
Inversely proportional to the square of the velocity (as particle slows down its energy loss increases)
Independent of mass of the charged particle (rate of energy loss of proton and electron at same velocity will be similar)
If the rate of energy loss of a charged particle is independent of mass then why do electrons not have a bragg peak on their depth dose curves?
As electrons will scatter and many end up travelling in the direction which they came from. Protons are heavier so not as easily deflected.
Energy loss per unit path length by a charged particle=
Mass collision stopping power (through hard and soft collisions) + mass radiative stopping power (through bremstrahlung)
What affects the mass collision stopping power of a charged particle?
Electron density of material (lower -> less collisions)
Energy of incoming particle (more energy lost by low energy particles)
What affects the radiative stopping power of a charged particle?
Proportional to:
Square of atomic number of material
Energy of incoming particle
What is LET?
The rate at which energy is imparted to the medium along a charged particle track (keV/um)
Also known as restricted linear stopping power as equal to collision stopping power after excluding secondary electrons with energies larger than a value. Draws attention to energy lost by electrons that is absorbed in close vicinity to electron path rather than the total energy dissipated by the electron (removes some delta rays)
What effects LET?
Particle type - alpha particle LET=50, 10kev electron LET=2.5, 1MeV electron LET=0.2
Particle energy - high energy particles have low rate of energy loss and small amount of ionisations, as particle loses its energy LET increases.
Describe the interactions neutrons undergo?
Elastic scattering- neutron interacts with nucleus as a whole. Nucleus gains kinetic energy and recoils through medium. Original neutron loses energy and is deflected from its path.
Inelastic scattering- occurs when a neutron is absorbed into a nucleus -> nucleus will be unstable and several different phenomona can occur:
- eject neutron
- eject proton/alpha particle/large nuclear fragment -> high LET
- eject a high energy photon
At what energies can neutrons cause radioactivity in a linac
> 8Mev (binding energy of nucleon)
What is the range of a positively charged heavy particle equal to?
Distance to bragg peak
How do you get a clinically useful proton beam?
Use various different energy beams to form a spread out bragg peak (SOBP)
Are protons high or low LET radiation?
Low LET radiation with a tiny high LET portion at terminal track
How are protons made?
Cyclotron or synchotron
Hydrogen gas + heat -> plasma + electrostactic force -> protons
What is absorbed dose and what are the units?
Mean energy deposited (dE) in a medium of mass (dM) by ionising radiation
dE/dM
What you prescribe 1 Gy = 1 joule/kg
Relevant to all ionising radiation- directly and indirectly
What is exposure and what are the units?
Number of ionisation events measured as an indication of deposited energy in air.
E = Q/m in Coloumbs per kg (C/kg)
Q is total charge of ions produced in air when all electrons liberated by photons in air of mass (m) are completed stopped.
ONLY APPLIES TO PHOTONS
Exposure is the amount of charge liberated per kilogram of air by an X ray beam – i.e. the number of ionisations of air particles
When can exposure be used?
ONLY PHOTONS ONLY IN AIR NEED ELECTRONIC EQUILIBRIUM couloumbs per kg Used in ion chambers
How do ion chambers measure dose?
Measure exposure
What is kerma?
Describes the photon transferring its kinetic energy to an electron -> energy transferred from indirectly ionising radiation to directly ionising radiation. how many electrons are activated by a photon beam in a given mass.
Kerma = Etr/m
Etr is sum of initial kinetic energies of all charged particles liberated by uncharged particles in mass
APPLIES TO PHOTONS IN ANY MEDIUM (not just air like exposure)
What are the units of kerma?
Joules/kg
What does KERMA take into account
Energy transfer from photon -> electron
At a single point
Interactions occurs IN MEDIUM (even if ionisation particle leaves the volume)
J/kg
What can KERMA be divided into?
Collisional kerma- kinetic energy expended in inelastic collisions (ionisation and excitation) with atomic electrons
Radiative - kinetic energy expended in radiative collisions with atomic nuclei (Bremstrahlung)
For low-energy photons, kerma is numerically approximately the same as absorbed dose. For higher-energy photons, kerma is larger than absorbed dose because some highly energetic secondary electrons and X-rays escape the region of interest before depositing their energy. The escaping energy is counted in kerma, but not in absorbed dose.
What is the difference between kerma and exposure.
Air kerma is an expression of exposure in terms of energy rather than charge
Exposure can be measured directly whereas kerma canot
Air kerma (j/kg)= exposure (C/kg) x Wair/e (J/C)
Wair/e =average energy required to produce an ion pair in dry air (33.97 J/C)
What is particle fluence?
Number of particles (N) incident on a sphere of cross sectional area (a)>
= N/a SI units M^-2
Indepedent of radiation direction
What is energy fluence?
Energy carried by these particles
Radiation energy (R) entering a sphere of cross sectional area (a)
R/a
SI units J m ^-2
Where is KERMA at its maximum?
At surface as most photons at surface.
Photons attenuated by the medium to kerma decreases with depth
Where is absorbed dose at its maximum.
Dmax (built up region prior to this)
Electrons continue to travel a short distance before depositing their energy
WHat does absorb dose take into account?
ONLY energy absorbed inside the medium
What is charged particle equilibrium?
Energy leaving the material = energy entering the material
Can assume absorbed dose = colllisional kerma
For energy <300Kv can assume radiative losses negligible however above this we cannot. Closest we get to CPE is at dmax. Beyond this kerm and dose curves divide depending on beam energy and transient charged particle equilibrium exists (TCPE)
What affects KERMA?
photon energy
atomic number of material
electron density of material
Across an interface between two materials how does kerma and absorbed dose change?
Change in kerma is proportional to mass energy transfer coefficients
As kerma changes so does the absorbed dose, according to the ratio of the mass energy absorption coefficient of two materials
Kerma will change in a discreet step
Absorbed dose will change more gradually due to backscatter (example of electronic disequilibrium)
What are mass energy absorption coefficients?
Energy absorbed per unit mass in two different materials subject to same photon fluence will be proportional to their mass energy absorption coefficient- ratio:
(μen/ρ)med/(μen/ρ)air
For materials with low atomic number the mass energy absorption coefficient does not vary much with energy. For materials with higher Z it is higher a lower energies where photoelectric effect is more probably and as increases it drops as compton scatter becomes dominant interaction and electron denisty become more important
How is radiation detected?
Heat (calorimetry)
Light (scintillator)
Ions -> electron current (ion chambers)
Electron/positron pairs (diodes) -> electric current
What is calorimetry and what type of dosimetry is this?
Change in temp determines absorbed dose Absolute dosimetry Energy J = mc (T2-T1) Mass (m) in kg c = Specific heat capactiy of medium (J/kg per centigrade) T1, T2- initial and final temp Therefore dose (gray) = c (T2-T1) Absolute detector Not used in hospitals as very large - but is used by the NPL as gold standard detector Graphite used as low heat capacity so heats up a lot for a small amount of radiation.
What gas detectors do you know?
Free air ionisation chamber
thimble ionisation chamber -> Farmer
Paralell plate chamber
Geiger muller counter
Describe a free ionisation chamber?
Photons enters air volume (metal box) and interact to form secondary electrons.
Electrons travel to anode, positive ions to cathode and electric current is measured.
Electrode separation must be sufficient that electrons completely stop in air (cause ionisation) before reach (proportional to exposure) - E<250kev = 20cm, E>1Mev - several metres
Must known mass of air from which collecting electrons
Must have charged particle equilibrium to make sure exposure = kerma (enough built up to primary interaction area).
Charged particle equilibrium = particles in = particles out so kerma (photons to electrons) = absorbed dose/exposure (electrons to ions)
Absolute dosimeter - very large so again not used in hospital - only used by NPL.
How do you calculate the dose from a free ionisation chamber?
Dose (Gy) = energy to produce one ion pair x charge collected / charge of one electron x mass air (kg)
J/Kg
What type of dosimeter is a free ionisation chamber?
Absolute
Describe a thimble ionisation chamber, its components and how it works?
Small cylindrical chamber
Graphite wall = cathode.
Graphite has similar atomic number to air but density x 1000, produces an electron density similar to free air chamber- enough to achieve charged particle equilibrium
Photon comes from outside and hits graphite wall - turns to electrons which travel through inner air to reach the anode.
Anode- central wire - collects the electrons
Measurement volume 0.6cm ^3
Operate at 200-300v
Farmer ionisation chamber is one specific type of thimble ionisation chamber.
Small so used in hospital but needs calibration - not an absolute dosimeter.
What is a thimble ionisation/farmer chamber used for?
Calibration of linac
Quality control and commissioning measurements
How are farmer chambers calibrated?
Cannot be exactly sure of measurement bolumber so need to be calibrated against an absolute dosimeter (send to national institute for physics) who provide a correction factor
How do you calculate dose from a farmer/thimble chamber?
reading x calibration factor x temp/pressure correction factor x ion recombination factor
What are the advantages vs disadvantages of a thimble chamber?
+ ves : size, linear response to dose, energy response equivalent of air
-ves: not absolute, requires calibration, ion recombination must be corrected for, high spatial resolution measurement difficult due to chamber size (measurement volume too large for field size <4cm, inaccurate at steep dose gradients)
Describe a parallel plate chamber?
2 parallel plates (electrodes)- 2mm apart
Wide guard ring ensures no in-scattering
Allows measuring point to be much better defined in space and to have finer measurment resolution - this is important for electrons as their depth-dose curve alters a lot over 5mm (width of a farmer chamber) but wont change much over 2mm (parallel plate chamber)
Requires correction factors like thimble
When would you use a parallel plate chamber?
Electrons
Surface/build up measurements with photons
Describe how a geiger muller counter works?
Gas chamber with a high polarising voltage across electrodes (900v).
Central anode- high voltage
Ionisation of electrons -> ionise another gas molecule-> avalanche effect -> entire tube ionised for each single photon event
Measures dose rate not dose
Very sensitive
Used for radiation protection purposes - e.g. if you drop a brachytherapy seed, use this to try to find it - audible indication of dose rate
What are the advantages vs disadvantages of geiger muller counter?
Advantages- sensitive, real time measurement, indication of intensity, portable
Disadvantages- saturates at high dose rates so you get a “dead tube” whilst waiting for it to reset, CANT determine dose, energy or type of radiation
Describe how a scintillation counter works?
NOT a gas chamber
Scintillation crystal (often sodium iodide) used- photon hits crystal and causes an emission of light.
Photocathode converted each photon to an electron -> photomultipluer tubes multiplies electrons to create a cascade which is measured as current.
Measures dose rate.
Radiation protection.
What are the advantages vs disadvantages of a scintillation counter?
Advantages: can measure energy as well as number of photons.
Disadvantages: need airtight container for crystal, sodium iodide only picks up high energy gamma (No beta or alpha)
Basically used the same as Geiger counter
What are solid state detectors used for?
Relative dosimetry of electron and photon beams
In vivo dosimetry
Quality assurance measurments
Name some types of solid state detectors?
Silicon diode detectors
TLDs
Describe the components of a silicon diode detector and how it works?
Silicon crystal (semi-conductor) has 4 electrons in its outer shell -> forms covalent bonds with neighbouring atoms -> crystal lattice If crystal absorbs energy then to bonds can break and leave a free negative electron and a positive hole. Impurities added to the crystal increase number of electrons or holes- called doping. -> impurity has 5 outer shell electrons eg phosphorus- extra electrons -> negative charge -> n type -> impurity has 3 outer shell electrons eg boron -> extra positive holes -> p type Electrical field naturally formed over the depletion zone (no need to apply high voltage) Ionising radiation produces electron hole pairs -> move towards electrons-> current Current proprional to ion pairs proportional to dose
What are the advantages vs disadvantages of silicon diode detectors?
Advantages
- very sensitive as very dense
- small 0.3mm3 measurment volume- good resolution for small fields
- no voltage required - hence can be used for in vivo dosimetry - used for PDDs in patients
- instant read out
Disadvantages
- poor tissue equivalence as atomic number is very different to tissue (much higher) so get more photoelectric effect so overdetect low energy photons
- over response to low dose radiation due to photoelectric effect
- gradual radiation damage and loss of sensitivity
- dose rate dependent
- not as reproducible as ionisation chamber
- relative dosimetry
- sensitive to temp- keep st skin temp
What are TLDs and how do they work?
Thermoluminescent dosimeters (TLDs)
Small chips of solid state materials.
When irradiated crystal absorbs energy and free electrons migrate in the lattice -> get caught in traps.
Later TLD heated and trapped electrons gain enough energy to be released and recombine with the positive hole -> visible light is emitted.
TLD reader converts the light into an electrical signal using a photomultiplier tube
So irradiate then heat - produces light - proportional to radiation dose
What are the advantages and disadvantages of a TLD
Advantages - small (tiny) - no cable or voltage - detects wide dose range - reasonable tissue equivalent - in vivo dosimetry if you don't need result immediately - radiation detectors - in detectors worn by staff for 3 months and sent off to be read Disadvantages - time delay before readout as need to warm up first - readout and calibration time consuming - delicate, small and easy to lose - accuracy limited at 5%
What is an array?
dose measurement device consisting of many ionisation chambers or diodes. Allows measurement across a 2D grid.
So can measure the beam flatness/symmetry.
When would you use an array?
Beam quality assurance
MLC accuracy
IMRT verification
Disadvantage: expensive, resolution inferior to film.
Name 2 chemical detectors and the differences between them?
Radiographic film (film goes darker) Radiochromic film (colour change)
What is radiographic film?
Thin flexible plastic sheet coated in radiation sensitive emulsion (eg silver bromide).
Radiation causes ionisation on film-> causes darker.
Need a dark room and optical densitometer to measure
Used in QC esp stereotactic radiotherapy
What are the advantages and disadvantages of radiographic film?
Advantages: - good spatial resolution - permanent record Disadvantages - need dark room - single use - delayed readout - calibraiton required
What is radiochromic film?
Changes in colour on exposure to radiation.
Colour change caused by polymerisation of dyes embedded in an emulsion layer coated on a substrate
Measured using a densitometer
Takes 6hrs to develop
Darker = more dose.
Used in commissioning new machines
Advantage: Very good spatial resolution.
Disadvantage: expensive and single use.
What detectors are used for absolute dosimetry and reference dosimetry?
Absolute:
kV- free air chamber
mV- calorimeter
Reference
- Farmer chamber
- parallel plate- electrons/brachy
What is the calibration chain?
Primary standard (nationally maintained)
Secondary standard - local standard used to calibrate the tertiary/quartenary equipment
Tertiary equipment- farmer chambers
Quartenary equipment - TLDs, diodes
What is the energy of superficial and orthovoltage XRs?
Superficial 50-160kV (10-100kv)
Orthovoltage 160-300kv (100-500kv)
Why can’t we use HVL to describe MV energies and what do we use instead?
Half-value layer (HVL) is the thickness of a material required to reduce the air kerma of an x-ray or gamma ray to half its original value
Not suitable as it is slowly varying function of energy and it can be affected by pair production at high energies.
Use water to specify beam energy:
- Tissue phantom ratio/quality index
- HVL and narrow beam attenuation coefficients in water
- PDD
What is percentage depth dose?
usually defined on central axis of beam (CADD)
- a quotient of the absorbed dose at that point, divided by the absorbed dose at the depth of max dose.
PDD = Dd/Dmax x 100%
In a MV photon what contributes to surface dose?
Collimator scatter
Phantom scatter- backscattered photons from within pahntom
High energy electrons produced by photon interactions in air
In a MV photon what is happening in the build up region and at Dmax?
Electronic disequilibrium
Secondary electrons travel downstream from the build up region and are NOT replaced by electrons generated further upstream
At dmax reach electronic equilibrium
Why is dose highest at surface in kV energies?
Electrons deposit energy where the photon interaction occurs - as low energy so don’t travel far.
Therefore kerma = absorbed dose
After d max why does the absorbed dose decrease?
Attenuation
Inverse square law
Both affect photon fluence
When the relationship between kerma and absorbed dose is constant what exists?
Charged particle equilibrium
ON a graph measuring kerma and absorbed dose against depth, what must the area under the curve for each of these be?
Equal
What factors affect PDD?
SSD
Beam energy (higher energy = lower surface dose, deeper D max, and slower drop off after D max)
Field size
As SSD increases what happens to effect of the inverse square law?
Effect decreases
Larger SSD = slower drop off, deeper Dmax (due to less lower energy scattered photons/electrons contamination) and lower surface dose (as less scattered photons/electrons contamination
If you have a long SSD is your PDD curve going to be higher or lower than if you have a short SSD?
Higher as less effect of inverse square law and it is percent (the actual dose would be higher if nearer)
If the dose-rate in a 6MV beam is 600cGy/minute at dmax for 100cm SSD calculate the dose rate at dmax for an extended SSD of 130cm (dmax=1.5cm)?
(101.5/131.5) ^2 x 600 = 357 cGy/min
What effects beam energy?
Attenuation
As energy increases the attenuation decreases- > increased photon fluence -> increased PDD at depth
What affects the attenuation coefficient?
Probability the photon will interact
Decreases with increasing energy
How does field size effect dose?
Bigger field size -> more scatter (scattered electrons and low energy photons), higher surface dose, deepest DMax at 5x5cm (smaller is less due to phantom scatter, larger is shallower is due to collimator scatter)
Magnitude of effect more pronounces at kV energies as scatter in all directions and therefore contributes more to central axis dose. At higher energies scatter is forward.
When can we not use PDD and what can we use instead?
Can’t use for isocentric treatment as no fixed SSD. Instead have a fixed SAD (source axis distance).
PDD is a function of SSD (varies depending on pt contour and gantry)
Instead use TPR (tissue phantom ratio)/TMR (tissue maximum ratio: dose at depth/dose at dmax.)
What is Tissue phantom ratio and tissue maximum ratio?
TPR: used to calculate relative dose at a given depth in isocentric treatment.
- ratio of dose delivered at depth d to dose delivered at reference depth dref at same SAD but surface of phantom has moved eg TPR 20/10 - ratio at 20cm and 10cm
Tissue maximum radio = where the reference depth is dmax
What is the output factor?
Describes what happens when you change the field size
10x 10cm field size the output factor = 1
OF = dose at dmax for field size of interest / dose at dmax for reference field size
Takes into account
- collimator scatter
- phantom scatter
How can you measure a beam profile?
Ion chamber
Diodes
Describe how the beam profile of a photon changes as it moves through a material?
photons that pass through the middle of a flattening filter go through more metal and are hardened so will deposit more dose more deeply.
Means at shallow depths- dip in centre of beam and as moves further this reverses.
What is the transmission penumbra?
Variation at edge of beam due to collimator thickness
What is the physical penumbra?
Lateral distance between 80% and 20% dose at isocentric plane at depth
What is the geometric penumbra?
Penumbra at any depth due to the geometry of the set up
What effects the geometric penumbra and how?
Focal spot size and shape - source size -> bigger source size the bigger the penumbra
SSD- increase in SSD increases the penumbra
Shape and properties of collimator- increase in SCD decreases the penumbra (further down/away collimator the smaller the penumbra)
Geometric penumbra originates from the radiation source when it is not a single point.
Is penumbra affected by field size?
no
Why use a wedge?
- compensate beams from non- orthogonal (right) angles
- compensate for changed in surface shape
- compensate for changes in depth dose fall off
What is the wedge angle?
Angle between the wedge isodose lines and line perpendicular to the central axis of the beam.
Should be defined at depth of 10 cm
What are blocks?
Customised blocks (cast from low melting point alloy-lead)- fitted in the linac head which shield critical structures.
