Physics Flashcards
What 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
EZ^2
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 (1/E)
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 squared (Z^2/E)
Z squared
Probability of Rayleigh scatter decreases with…
increasing incident photon energy (Z^2/E)
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 (E cubed) (Z^3/E^3)
Photoelectric absorption increases with
Atomic number, z cubed (Z^3/E^3)
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 and by electrons moving between atomic shells (characteristic X rays)
Gamma rays are emitted from nuclei 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 voltage, higher energy XRs produced
Higher charge through cathode = higher intensity
Higher Z number of target = higher intensity
What is attenuation?
Loss of photons as beam penetrates some materials. (Compare that to charged particles: photons lose number of photons so are ATTENUATED, charged particles reduce in energy so think about energy transfer/stopping power)
= 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?
Effective atomic number is 9
<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 for charged particles?
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) - not sure that is true - think higher mass = higher linear energy transfer.
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
Inversely proportional to square of mass hence only occurs with electrons and positrons
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.
LET increases with mass andcharge
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)
kinetic energy released per unit mass
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
ρ = density
μ = linear energy absorption coefficient
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 = photons
Paralell plate chamber = electrons
well-type ionization chamber = brachytherapy
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
temperature and pressure correction factor = room temp in kelvin (+273) / 293 x 1013 / room pressure in mbar
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 -> electrons move towards n-doped region (as the edge of it is deplete of electrons) and "holes" move towards p-doped region = 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
Not used anymore
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.
Very high resolution so used in SABR verification (Array is too low resolution)
What detectors are used for absolute dosimetry and reference dosimetry?
Absolute (primary standard):
kV- free air chamber
mV- calorimeter
Reference (Secondary and tertiary standard)
- Farmer chamber
- parallel plate- electrons
- well type - brachytherapy
Relative dosimetry (quaternary)
- TLDs
- solid state
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 intensity of an x-ray or gamma ray to half its original value. It is used to describe beam quality of kV photons. Quality described by TPR 20/10 for MV photons and RD50 for electrons.
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,
Small effect with larger SSD: deeper Dmax and lower surface dose as less low energy scattered photons/electrons as they don’t get that far
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
How does energy affect PDD of photons?
Lower surface dose, deeper D max (1/4 of energy) and slower fall off - as less attenuation (but inverse square law component stays the same)
(Lower surface dose relative to Dmax as Dmax is prportionally higher with higher energies)
As energy increases the attenuation decreases- > increased photon fluence -> increased PDD at depth
What affects the attenuation coefficient?
Probability the photon will interact
Properties of material: density, Z, electron density.
Properties of photon: quality
Decreases with increasing energy
How does field size effect dose?
Bigger field size -> greater proportion of dose from scatter (scattered electrons and low energy photons and scattered photons from sides), higher surface dose (more scattered electrons and photons through collimator), shallower fall off (more phantom scatter coming into midline)
(Small effect: dmax is deepest at 5x5cm although mostly stays the same: smaller than that causes less phantom scatter so less inward or backward scatter, larger than that increases electron surface scatter so brings dmax to the surface)
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?
Arrays:
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 photons travelling through the collimator
What is the physical penumbra?
Lateral distance between 80% and 20% dose at isocentric plane at depth
Made up of transmission penumbra (transmission attenuated through collimators) + geometric penumbra (As source has a size) + scatter penumbra (inpatient scatter outwards)
What is the geometric penumbra?
Penumbra at any depth due to the geometry of the set up (due to the finite size of the X ray source of focal spot size)
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 (missing tissue externally e.g breast)
- compensate for changes in depth dose fall off (missing tissue internally e.g. lung)
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.
What are the advantages and disadvantages of blocks in radiotherapy?
Advantages:
- simple
- good penumbra
- excellent spatial resolution
Disadvantages:
- heavy
- alloys can be toxic
- manufacturing time
- dose not zero under block - affected by field size, scatter from patient, block size and position, depth of interest
What dose is able to pass through MLCs?
Radiation transmitted through the leaf of material <1%
Radiation leakage between adjacent leaves - inter-leave leakage <2%
What are MLCs
2 banks of leaves, driven automatically but independent motors. Step or tongue and groove design to reduce interleaf leakage
What are the advantages and disadvantages of MLCs?
Advantages:
Quick
no manual handling
Mulitple field delivery
Disadvantages:
- leakage
- worse penumbra
- MLC direction sometimes limited by wedge direction
- conformity poor
What is a tissue compensator?
Block which can compensate for dose gradients perpendicular to the central axis (wedges can only provide compensation in one direction). They basically are inverses of patients to provide a uniform dose at the target.
- 2 dimensional, made from aluminium or brass blocks
- mounted at a distance from the patient as otherwise skin sparing lost
- causes beam hardening
- not used commonly now due to IMRT
What is the relative density of lung and bone in relation to water and how would this affect attenuation of a beam?
Relative electron density of lung =0.2-0.3, water =1, bone= 1.2
1cm water = 4cm lung = 0.8cm bone
What happens at the lung tissue interface?
As enters lung there is a slight reduction in dose due to reduced backscatter from the lung.
Total attenuation is less in lung due to reduced physical density which increases photon fluence distal to lung and increases dose beyond lung.
As re-enters tissue the dose builds up again
Bigger effect with smaller tissue sizes
To calculate dose distal and remote to the lung we can use a standard correction factor
Lung +3%/cm
Bone -2%/cm
What are isodoses?
Combination of depth doses and profiles to give a 2D representation of dose
What happens to the isodoses of a 6MV photon beam at depth.
Increase spacing between isodoses on central axis as depth increases- > reduction in gradient of depth dose curve
Widening of penumbra at depth
Increased rounding of the isodoses at depth
What is the minimum information required for a basic plan?
Size, position and depth of target
Beam data- field size, shape
Absolute dose calibration of machine (dose delivered in a reference set of conditions)
What are the reference conditions for monitor unit calculations?
100cm SSD
Reference depth = dmax
10x 10cm field
1cGy= 1MU
What is MU calculation for a fixed SSD?
100 x Dose (cGy) / PDD x OF (cGy/MU) x WTF x PbTF
What is the MU calculation for isocentric MU?
Dose (cGy) / TMR x OF (cGy/MU) x WTF x PbTF x (100/100 + dmax)^2
How do you work out equivalent square for a rectangle?
2ab/ a + b
How do you work out equivalent square for a circle/ellipse
0.9 x 2ab/ a + b
Describe depth dose distribution for an electron?
Small skin sparing effect but high dose near surface and rapid fall off with small tail due to Bremstrahlung (5% dose)
How do you calculate d max of an electron beam?
Energy x 2 = dmax in mm
How do you calculate R90, R50 and Rp (practical range) of electron beam?
Energy x 3 = R90 (theurapeutic range)
Energy x 4 = R50
Energy x 5 = Rp
What factors affect depth dose in electrons?
Energy - increased energy = higher surface dose, deeper dmax, deeper practical range (as with more energy there is less scatter so surface dose is more similar to dmax so plateaus and goes deeper so slower fall off, whereas with small energy there is a lot of scatter causing the build up region)
Field size (only effects if smaller than practical range) - generally surface dose decreases and dmax gets deeper and fall off is less step with increasing field size up to the practical range due to lateral electronic disequilibrium (think of it as you are only measuring the forward electrons so losing all the lateral electrons (so weaker beam) but get plateau for surface dose and dmax). Practical range stays the same. equivalent square should NOT be used for electrons. Electron output factors should be measured not estimated.
SSD- complicated as not physical source so inverse square law cannot apply. Define a virtual source. Increased SSD = lost weak/scattered electrons so beam effectively more energy = lower surface dose, deeper dmax, deeper practical range
Surface obliquity >30 degrees = brings PDD closer to skin so can get hotspots if heterogeneity in angle - need bolus to smooth out.
Homogeneities: have bigger effects on electrons than photons.
Describe the isodoses of electrons?
Beam flaring (scatter of electrons away from central axis) esp at lower isodoses Wide penumbra At higher energy the higher isodoses converge due to increased forward scatter
What is bremstrahlung?
Braking radiation
Proportion to Z^2 of material
Inversely proportional to mass of the particle (so protons don’t do it)
Proportional to charge of the particle.
Therefore electrons and positrons the most efficient at doing it.
Energy produced from deceleration of a charged particle that passes near to the nucleus
How do you calculate HVL
0.693 / u
u = linear attenuation coeffcieint
Inversely proportional to it.
The amount of material required to reduce the photon beam intensity to half its entrance value
Used for beam quality in kV beams. Homogeneity index = first HVL/second HVL - 1 for homogeneous beams, less than 1 for heterogeneous beams.
For an electron beam to change from percentage depth ionisation to percentage depth dose curves what do you do?
multiple by the water to air stopping power ratio
Tissue maximum ratio depends on…
Energy, depth and field size.
Affected by attenuation only, not inverse square law
Independent of SAD as fixed
What does surface obliquity cause and when does it become a problem for electron beams?
Cause stand off (gap to applicator)/stand in (within the applicator)
Increased obliquity = increased surface dose, shallower dmax, faster falloff.
This is because the electrons are produced in the same angle so don’t travel as far inside the patient as they are travelling at an oblique angle for electrons.
> 40 degrees it has more of an effect
What is the applicator cut out factor for electrons?
ACOF = ratio of measurement with an ion chamber for a 10x 10 cm field at dmax, 100cm SSD to a measurement using individual cutout at dmax
ACOF = output (10x10cm, dmax, 100cm SSD) / output (cutout, dmax 100cm SSD)
How do you calculate monitor units for electrons with a cut out?
MU = (dose/#) / ACOF
How do you calculate ACOF with kV?
ACOF = backscatter factor cutout/ backscatter factor applicator x applicator factor
How do you calculate MU using kV?
MU = (dose (cGy)/#) x (1/ACOF) x (dist/SSD)^2
What planning methods are used?
Dose kernels to represent the dose deposited from a beam of photons spreading out due to electron scatter.
Can be modelled by convolution (aka pencil beam) (1D) vs superposition (3D) - superposition is more accurate as takes into account lateral electronic disequiblibrium in different media so more accurate for heterogeneities such as lung to tissue transfer.
Superposition also called collapsed cone or AAA (brand names).
Deterministic = pencil beam, collapsed cone, AAA
Pencil beam - started off along lines of montecarlo but then uses much quicker methods
- uses small kernels created in advance during commissioning of TPS
- kernels are a map of the dose that will occur for your energy of radiation beam eg 6MV hitting small volume of water
- scatter form any patient can be recreated by adding up these pre-calculated small scattering volumes
- process called convolution
Non deterministic (based on probabilities) = monte carlo method. Probabilistic modelling of billions of electron interactions - most accurate.
What is the GTV?
Gross demonstrable extent of disease on imaging or examination.
What is the CTV
Includes subclinical microscopic disease that has to be treated to adequate dose to achieve cure/palliation. Based on clinical knowledge/trials and specific to each tumour.
What is the PTV
Includes geometrical margin for set up error and organ motion (internal margine and set up margin). Akin to a hoop to catch the tumour with.
What is the internal margin
Accounts of for changes in size and position of CTV
IM + CTV = ITV
What is set up margin
Accounts for uncertainties in relationship between person and beam
What dose does the ICRU recommend the PTV is covered by?
95-107%
What is the treated volume?
Volume of the 95% isodose
Would ideally be equal to PTV but usually higher
What is the irradiated volume?
Volume of tissue that receives a dose that is considered to be significant in relation to normal tissue tolerance
What is the PRV?
Planning organ at risk volume
OAR move and are susceptible to the same random and systematic errors as CTV so we grow the OAR volume by the set up amount to create a planning organ at risk volume.
What is the ICRU reference point?
A point chosen within the PTV defined as:
- dose at point should be clinically relevant
- point should be easy to define in a clear and unambiguous way
- the point should be selected so that the dose can be accurately determined
- the point should be in a region where there is no steep dose gradient
Ideally the point should be isocentre but not always possible
What is the minimum level of dose reporting that should be carried out?
Level 1 reporting (not good enough for IMRT):
ICRU reference point dose
Max dose to PTV
Min dose to PTV
Dose to OARs
Tumour control depends on dose to CTV, however can only be estimated by dose to PTV
What is included in level 2 reporting?
Used with computerised planning GTV CTV OR PTV PRV Prescription to PTV mean or median (D50%) rather than ICRU ref point for IMRT as there can be significant heterogeneity Report PTV min and max dose using D2% and D98% Advanced treatments like IMRT - DVH
What is pitch of a CT?
Pitch = table travel per rotation/ XR beam width
eg
travel = 10mm/rot
BEam = 10mm
Pitch = 1
How is each pixel of a CT labelled?
Each pixel has a CT number
CT number is in Hounsfield units.
What are hounsfield units?
Given information about the way the XR beams are attenuated in material
HU = μtissue - μ water / μ water x 1000
μ = linear attenuation coefficient
What would be the CT number for bone, muscle, water, fat and air?
CT number in HU Bone + 1000 - white MUscle 10-40 Water 0 Fat - 50 to - 100 Air -1000 Black
How do treatment planning systems calculate dose?
Requires the electron density to calculate dose
Uses CT number/HU to calculate electron density
A calibration curve defines the relationship between CT number and electron density measure on CT scanner
What is windowing of a CT
Used to change the greyscale on XR image
Out eyes can only distinguish 40-100 types of grey and the system has 256 levels
What is the typical dose of CT Head, CXR, CT abdo/pelvis?
CT head 2mSv
CXR 0.02 mSv
CT abdo/pelvis 20 mSV
How do we plan with contrast?
Contrast falsely elevates CT numbers
Dosimetrist can override CT numbers of TPS
What are DRRs?
Digitally reconstructed radiographs
Images made from reconstruction of planning CT
Represent same anatomy as used for planning
Can be compared to kV or MV image during set up to check position.
What techniques are available for motion control during radiotherapy?
DIBH - prospective gating
- can monitor this automatically and set beam to only be on when in deep breath hold. Can use optical tracker on patients chest
4D CT - retrospective gating
- acquire moving image of tumour.
- each CT slice is timestamped and correlated with pateitn breathing phase
- 4DCT volumes = MIP (maximum intensity projection), MinIP (minimum intensity projection), AIP (average intensity projection)
What is DICOM?
Digital Imaging and communications in medicine
A standardised way of storing and transferring an image with attached patient information
Uses a set file format definition and communications protocol
What are common artefacts in CT imaging?
Streak artefacts- caused by highly attenuating materials eg prosthesis
PArtial volume effect: when high contrast object is smaller than the slice thickness, the detector averages over the radiation intensities in z direction. Therefore size of object may be over estimated and its CT number underestimated
Ghosting : caused by patient/organ movement
What is coplanar planning?
One in which the central axes of the beams lie within the same plane - if restricted to a 2D model then has to be coplanar. e.g. the arc of VMAT can be in one plane so coplanar.
What is non-coplanar planning?
When beams lie in a different plane
Offers greater flexibility and allows improved dose distributions however often best solution in coplanar is it will minimise path length
How does TPS make isodose display?
Calculate dose to a 3D series of points called a dose grid
Join them to make isodoses
Can be used to generate DVHs
What is a DVH? What 2 types are there?
2D representation of 3D dose distribution for individual organs. Does not represent full isodose distribution as does NOT contain geometric information
- Differential (frequency) DVH
- Integral (cumulative) DVH
What is a differential DVH? When is it used?
Also called frequency
Fractional volume of organ recieving a dose
Shows DOSE HOMOGENEITY to a structure
Useful for looking at PTV as can easily see width of peak and max/min doses (narrower better)
What is a integral DVH?
Also called cumulative
Fractional volume receiving that dose of greater
Used more, especially for OAR
Used to see the global max received by a serial organ eg spinal cord
What is forward planning vs inverse planning?
3D conformal treatments are generally forward planned -> planner decides all treatment parameters (field size, beam weight, gantry angle etc) then sees what dose distribution is.
Inverse planning- computer makes changes and decided if it likes it based on criteria the planner has set with the max/min acceptable doses -> computer attempts to find solutions
What is IMRT?
Allows us to create a curved isodose distribution
In order to do this need to modulate the radiation field in 2 dimensions -> use moving MLCs
What is VMAT?
While beam is on gantry rotates.
Brand names: RapidArc (Varian), VMAT (Elekta), SmartArc (Pinnacle TPS)
Volumetric Modulated Arc Therapy (VMAT)
We can vary: gantry rotation speed, dose rate and MLC position
What is tomotherapy?
Based on CT scanner
Short 6MV waveguide spinning around patient which produces a fan beam of 6MV photons.
Modulated by mini MLC
Dose built in a helix with patient moving through gantry
What QC is required when getting a new TPS?
Calculation algorithms already installed but no configured
Done by local physics department
Everytime TPS software updated the physicists have to re-check everything
What QC is required for CT scanner?
Check CT calculating electron density correctly
Put phantom in CT with pieces of electron dense material separated by a known distance
What QC is required by physics for each plan?
Conformal plan - patient details, moves from tatoos to treatment centre correct, plan is good (covers PTV, avoids PRV), correct dose and fractionation, calculations correct, all local processes followed
VMAT/IRMT - all above, plus check TPS algorithm modelling MLC movement correctly so should check on plan on linac and have software to read this and compare.
Is a cathode positive or negative?
PANCAKE
Postive Anode
Negative Cathode
Describe process of making a clinically useful XR beam?
Evacuated (glass/ceramic envelope)
Cathode - filament tungsten wire
- electrons boiled off in thermionic emission - intensite dependent on heat/electric current
Negatively charged foccussing cup - direct electrons to small are on target
Anode - target - tungsten
- +ve to attract electrons
- high z for efficient XR production
- high melting point
- copper stem
Filter - beam quality
Collimation
- hooded anode provided initial collimation (blocks stray electrons)
- applicator at fixed distance
What is the beam profile of kV XRs and why does this occur?
Significant drop off from of intensity from central axis of beam towards edge
Due to HEEL EFFECT
- xrs come off anode at different angles, if they come off at more of angle then have to pass through more material so more attenuated and lose more energy as heat.
How is the output controlled in kv machine?
Either a timer or an ionisation (monitor) chamber is used to control dose delivered
After set time delivered voltage current turned off but most use monitor chamber
What is a cobalt 60 machine?
What are the advantages vs disadvantages of it vs linac?
Historic machine when technology could only produce low energy x rays. Called teletherapy. Cobalt60 is artificially made.
Produces high energy gamma rays (compared to the x ray energies being produced at the time)
- half life 5 yrs (better than Radium which is the naturally occurring radioactive material previously used).
- forward scattering produces relative skin sparing effect with max dose at 0.5cm
- simple method, no need for high skilled maintence - can be used in developing countries
- source is 2x 2cm long cylinder - large source gives large penumbra so requires primary and secondary collimator then penumbra trimmers
- dose rate decreases over time and have to replace every 5 years
- has radioactive source so always on - radio protection problems when decommissioning or replacing source
- low energy compared to LINACs so harder to get higher dose treatment (although their spectrum is different as it is only peaks rather than a spectrum so the beam quality is higher than expected from the beam energy compared to X ray beams)
Describe process of electron generation in a linac?
Electron gun = cathode = tungsten - heated = thermionic emission - thermally induced flow of electrons is produced from hot cathode
Within cathode cup
Anode at entrance to waveguide
Electron current into waveguide - controlled by filament current and hence filament temperatures
Operates in a vacuum to prevent collisions with gas molecules
Which part of the linac is a vacuum and how is this maintained?
Operates in a vacuum to prevent collisions with gas molecules
- electron gun, accelerating waveguide, transmission waveguide, beam transport all occur in a vacuum
- vacuum maintained using an ion pump
- interlocks prevent the use of the linac if the pressure in these areas exceeds a predetermined value
Describe what is used to accelerate electrons in a waveguide?
Electrons accelerated under action of microwaves
MAGNETRON
- Radiofrequency oscillator, created microwaves from accelerating electrons in magnetic field
- 3 Mwatts (energies up to 10MV)
- physically smaller
- used in low energy linacs
KYLSTRON
- Radiofrequency amplified
- amplifies low power microwaves
- 7Mwatts
- mounted in insulating oil in stand
Microwave generators cannot operate continuously at high power - so pulses
Describe the components in and around a waveguide?
Initially electrons experience different degrees of acceleration and become bunched. Then reach speeds close to speed of light.
Waveguide can be travelling or standing waveguide - either way carried like a surfer on a wave.
Enter and exit - steering coils
Middle - focusing coils - Solenoid
- electrons tend to diverge as they travel through waveguide as repel each other
- solenoid focussing coils outside waveguide, cooled using chilled water system
Water cooling
- accelerating waveguide, microwave generator, sterring/focussing coils, XR target all require cooling by chilled water
- linac cannot be used if no chilled water supply
What is required if the waveguide is perpendicular to treatment head?
Waveguide can be inline or perpendicular to treatment head
Achromatic bending magnet - all energies kept.
If perpendicular then need to bend electrons with bending magnet - this helps to focus electrons
When electrons exit the bending magnet describe their path through a linac (for MV)?
- Target (tungsten - high z)
- Primary collimator
- Flattening filter (not required in IMRT)
- Monitor chamber
- Mirror - field light
- Secondary collimator
- Multi leaf collimators
Describe the make up of a monitor chamber?
3 segments - Ch1, Ch2, & segmented chamber
Ch1 = primary channel, terminates the beam once the required dose has been given
Ch2 = back up if Ch1 fails
If that fails there is also a timer
Segmented chamber- monitors beam characteristics - uniformity and symmetry.
The uniformity can be used to control the gun filament electron emiussion
The symmetry signal can be used to control the steering coil currents
What is the field light?
Optical representation of radiation field size
Light from bulb is reflected onto the secondary collimators from behind
Reflecting mirror - thin foil, transparent to radiation
Can aid set up
Matches 50% isodose
Tolerance 2mm
Crosshair/wire represents centre of radiation field
What is the ODI on a linac?
Optical distance indicator
Bulb lights a scale that indicates distance of the patient from radiation source
What are the secondary collimators made of and how do they work?
4 thick tungsten or lead alloy blocks e set of opposing jaws mounted above each other Labelled x and y jaw Can be moved to define rectangular field Move independently so can be asymmetric Usually parallel with beam to reduce transmission penumbra
What has to be removed and inserted to change a linac from MV beam to electron beam
Move out:
- target
- flattening filter
Add:
- scattering foil
- applicator
What is the scattering foil made of for electron beams?
Thin foils of high z material (Cu or Pb)
- High z increases scattering
- thin to reduce XR contamination
CAUSE BREMSTRAHLUNG
What is applicator made of for electrons?
Made of low z materials, collimates beam without generating unwanted XRs
How is the patient aligned when get on couch?
2 sets of indicators
- one lines up the axes of radiation beam and hence radiation beam -> field light + crosswires
- 2nd indicates location of isocentre of machine -> optical distance meter at 100cm, room lasers
Surface marks on patient allow them to be set up relative to isocentre
How does the couch move and what is couch indexing?
Isocentric rotation - allow target volume within the patient to be orientated to the maching.
6 planes: 3 rotational and 3 translational planes (forward/back, up/down, left/right, yaw, roll, clockwise/anticlockwise)
Couch indexing where the patient alignment devices lock into holes on couch - allowing quick, accurate set up.
What is inter-fractional motion and how is it overcome?
movement inbetween fractions
e.g. anatomical changes due to weight loss H+N or pleural effusions (lung) - review and may need replanning
Rectal/bladder filling - use enemas and set bladder filling protocols/ultrasounds
Immobilisation e.g. shells, vac bag, tattoos to keep patient in same position
Imaging used to compare to planning CT or DRR or marker seed lining up e.g. kV, cone beam CT, portal imaging (MV)
Dealt with using standard positional verification systems - mainly EPID systems using bony landmarks or markers
What is intra-fractional motion?
During treatment movement
e.g. breathing (DIBH in breast or 4D CT in lung SABR or gating or abdominal compression in liver SABR to only allow shallow breathing), heart pumping, rectal/bladder filling (try not to take too long), patient moving (immobilisation),
What is stereotactic radiotherapy characterised by?
Extremely high geometric accuracy <1mm
Extremely high dose gradients - use well defined collimation systems and many beams/beamlets can spare nearby organs at risk - drops off at 25% per 1mm
Only effective for small volume disease
Uses Flattening Filter Free beams
Where do you define the dose for stereotactic radiotherapY?
Periphery of the target
What is stereotactic radiosurgery?
Single fraction of an ablative dose, intracranially
Prescribed to a low isodose 50-85%
What is stereotactic radiotherapy (as opposed to stereotactic radiosurgery)?
Fractionated stereotactic radiosurgery typically delivered in 3-5 doses usually intracranially and prescribed to low isodose 50-85%
What is stereotactic ablative body radiotherapy (SABR) or stereotactic body radiotherapy (SBRT)?
Fractionated radiotherapy delivered extra-cranially in 3-8 fractions, usually prescribed to a higher isodose (60-80%)
Describe how a SRS gammaknife works?
Head frame - drilled into skull
Newer ones use CBCT
Uses lots of small cobalt sources - around 200 - one for each collimator
Head placed into hat with tiny holes which are used as secondary collimators
Patient is moved so target is at centre of all tiny beams
MRI based planning
Deliver in 1 day
Low dose rate (so I assume takes a long time)
Prescribe to 50% isodose
How does SRS work on a linac?
Old Linacs
- add extra fixed collimator to make beam smaller and more accurate
- still has flattening filter so beam more homogenous, cant get doses up really high
New linacs
- non-coplanar VMAT
- skull is frame of reference
- flattening filter free
Describe how cyberknife works for SRS?
6mV mini linac
COlimators produce circular field sizes 5-60mm diameter
Mounted on high precision robotic arm
Frameless system
Couples with high resolution stereoscopic kV imaging
Patient position is tracked using two orthogonally placced XR camaras
Non-isocentric
Live images taken from each XR before each beam compared to library of DRRs created from original planning CT
4 methods of tracking- bony skull, implanted fiducial markers, bony spine, lung tumour itself
Very versatile but treatment times long
When an element undergoes alpha decay, how does the atomic number and mass number of the new element change?
Atomic number decreases by 2
Mass number decreases by 4
What is a decay series?
Series of steps radioactive materials undergo until reach a stable state
What is beta minus decay?
How does the mass and atomic number change?
Neutron in nucleus is converted into a proton and electron.
Electron + anti-neutrino emitted
Mass number - stays same
Atomic number - increases by 1
The max energy of beta particle released is the difference in the mass between the original nucleus + post emission nucleus, remaining energy carried by anti-neutrino
What is beta positive decay?
How does the mass and atomic number change?
Proton is converted into a neutron and positron
Positron + neutrino emitted
Mass number stays same
Atomic number - decreases by 1
Once released positrons lose their kinetic energy, when most of it has gone they combine with an electron. Their combined rest mass turns into 511keV photons travelling in the other direction from the annihilation - what happens in PET!!
What is electron capture?
How does mass and atomic number change?
Nucleus combines with one of the orbiting electrons, converting a proton to a neutron and releasing a neutrino
Mass number : same
Atomic number : decreases by 1
Loss of K shell electron so another electron drops down to fill its place emitting a photon/auger electron
Name an isotope that decays using electron capture?
I-125
When are gamma rays release from nuclides and when are XRs release?
Both alpha and beta decay process may leave nucleus energetically unstable -> release gamma rays
Internal conversion - in contrast to gamma decay, an energetic nucleus can sometimes release its excess energy to an orbiting electron (usually k shell) -> electron escapes -> vacancy filled by higher energy electron -> photon/auger electron
What is radioactivity measured in ?
Becquerels
1Bq = 1 disintegration per second
Or curies
1 curie = 3.7 × 10^10 Bq
What is half life and what is it dependent on?
The time for radioactive material to lose half of its activity
Dependent on number of nuclei present
What is T1/2 equal to?
ln2 / λ
λ = decay constant
What equation do you use to calculate amount of a radionuclide remaining after time T?
N = N0 x e ^ - λt
N = amount after time t N0 = amount at start λ = decay constant t = time
Name three ways we can artificially produce radioactive materials?
Fission
Neutron bombardment
Charged particle bombardment
What is fission?
Occurs in a nuclear reactor
Splitting of a large atom into roughly two equal parts
Neutron enters nucleus, making it unstable and results in it splitting
Produces Strontium 90
Tellurium 131 decays to iodine 131
What is neutron bombardment?
Stable element placed in nuclear reactor
Bombarded with neutrons -> rearrangement and release of gamma rays
Makes beta negative decay products cobalt 60, technitium 99
What is charged particle bombardment
OCcurs in a cyclotron - more expensive
Makes beta positive decay products
Stable element bombarded with protons or alpha particles
Leads to absorption of one and ejection of one or more neutrons
Makes carbon-11, fluorine 18
What is radioactive equilibrium?
Occurs when rate of production of a radioisotope is equal to its rate of decay and so its quantity remains constant
For this to occur the half life of the parent has to be greater than the half life of daughter product
What is interstitial brachytherapy?
Implanted directly into tumour
What is LDR brachy?
0.4-2Gy/hr
What is MDR brachy?
2-12Gy/hr
What is HDR brachy?
> 12Gy/hr
What are the advantages of brachytherapy vs EBRT?
Rapid fall off dose
High dose directly to tumour and relative sparing of normal tissue
No need for margins to account for organ mortion
Short - treatment time - reduces repopulation
Can use when limited by normal tissue tolerances
What are the disadvantages of brachytherapy vs EBRT?
Most common cause of litigation in oncology
Very high dose close to applicators
Difficult to salvage if applicators were not in right place
Operator dependent
Time consuming
Increased radiation risk
Costly
What is the paris system of dosimetry and how do sources need to be configured?
Developed to standardise the placement and dosimetry of iridium wire and hairpin implants
Sources need to be STRAIGHT, PARALLELL and of EQUAL LENGTH with separation 5-20mm
Dosimetry is calculated on the central plane midway between sources
When performing dose calculations in brachy how is source strength expressed?
Reference air kerma
On a specified date and time
What is remote afterloading in brachytherapy?
Applicators fixed to afterloader which delivers radiation without anybody in the room
What is a pellet afterloader system?
Source pellets and spacers are programmed and assembled in a source train.
Positive air pressure system forces pellets from safe into the applicators
Can be used for LDR, MDR, HDR brachy
Disadvantage: all pellets have to remain in an individual catheter for same times - not flexible
What are stepping source systems in brachytherapy?
Single stepping source system - moves the source to predetermined positions
What are the essential features of an afterloader?
Multiple channels can be connected to a variety of different applicator types
Thin and flexible to go around curves
Easily programmable
Direct plan transfer from planning system to treatment machine
What are the afterloader safety features?
Back up secondary timer
Automatic check of transfer tube system before source exposed
Built in source position checks
OPerating system check that source returned properly
Back up power supply
Source held in a safe so low dose around machine when not in use
Manual source return in event of a complete power failure
Automatic retention of treatmetn data and history in event of power failure
Alarm and status code systems to alert user of faults
When are sources for brachy checked?
Need to calibrated on reciept
Safe audits - stock checks monthly
Wipe tests annually
Documentation & disposal
What is air kerma rate
Kerma rate to air, in vacuo at a reference point 1m from the source centre
Units = μGy h^-1
What is air kerma strength
The product of air kerma rate at a distance d, measured along the transverse bisector of the source and the square of the distance
1 U = 1 μGy m^2 h ^-1 = 1 cGy cm^2 h^ -1
In cervical brachytherapy what planning volumes are defined?
GTVb - macroscopic tumour spread at time of brachy
HRCTV (high risk)- whole cervix and presumed extension of tumour
I- CTV (intermediate) - based on tumour prior to EBRT
OAR
Should report total reference air kerma, point A doses, IRU reference doses to points on rectum and bladder
What QA must be performed for HDR remote afterloading devices?
Pre-treatment - machine function tests (interlocks, emergency equipment, audio/visual systems)
- source data checks - verify date, time, source strength in planning computer
Postional accuracy - verify source position of stepping source with CCTV and ruler
Temporal accuracy with stopwatch
Applicator integrity - inspect for any damage
What is the half life of Iridium- 192?
74 days
What is the emission from Ir-192?
What is it used for and why?
0.38Mev (average) gamma rays
Brachy wires, pins
HDR brachy
Iridium 192/77 = platinum 192/78 + electron + photon
High specific activity
What is half life of I-131?
8 days
How is Ir-192 made?
Neutron bombardment
How is I-131 made?
Fission of uranium-235
What does I-131 emit?
Beta <606Kev
Gamma 364 keV
What is I-131 used for?
Thyroid, neuroendocrine tumours
What is I-125 used for ?
Brachy seed - prostate
LDR
WHat is half life of I-125?
59.6 days
What does I-125 emit?
Characteristic XRays (from electron capture): 27.4 and 31.3keV
Gamma rays: 35.5 keV
Iodine 125/53 + electron = tellurium 125/52 + photon
27.4, 31.3, 35.5 keV
How is I-125 made?
Decay produce of Xe 125
What is Sr-89 used for? (not in curriculum)
Target bone mets
What is half life of Sr-89? (not in curriculum)
50.7 days
How is Sr-89 and Sr-90 made? (not in curriculum)
Nuclear fission
What does Sr-89 emit? (not in curriculum)
Beta minus decay
583 kev (mean)
Degrades to Ir-89
What is the half life of Sr-90 (not in curriculum)
28.7 years
What does Sr-90 emit and how does it decay? (not in curriculum)
Beta minus decay - emits 0.546MeV
Sr-90 -> Ir -90 + electron + antineutrino
Ir-90 has half life of 64hrs and decay energy of 2.27MeV
How is cobalt 60 made?
Neutron activation / bombardment
What does cobalt 60 emit
Gamma
1.17 & 1.34 Mev
Cobalt60/27 = Nickel 60/28 + electron + photon
+ some beta decay
Used in HDR afterloader and gammaknife
What is the half life of cobalt-60?
5.26years
What does radium-223 emit? (not in curriculum)
Alpha particles 5000-7500kev
What is half life of radium-223 (not in curriculum)
11.4 days
When is radium-223 used? (not in curriculum)
Mets castrate resistant prostant cancer
Must have tried at leas 2 systemic therapies first
NO VISCERAL METS
Bone scan/CT confirming osteoblastic bone mets
IV injection
What is the half life of Y-90? (not in curriculum)
2.7 days
What is Y-90 used for? (not in curriculum)
Microspheres for radioembolisation of hepatic mets
What does Y-90 emit? (not in curriculum)
Beta minus decay to zirconium-90
2.28MeV - beta - travels 11mm in soft tissue
Emits small percentage of bremstrahlung XR photons for post therapy imaging
What is a sealed source?
Enclosed
Inserted and removed, no radiation left inside patient
Sometimes inserted and left in body in sealed iodine pellets eg prostate
What is an unsealed source?
Usually liquid
When inserted in body it disperses, patient is radioactive
What is the biological half life of radionuclide in a tumour or organ?
Time taken for the biological retention of radioactivity to reduce to half its original value
the time it takes for the concentration of a stable isotope to be reduced by one-half in a tissue or organ
What is the effective half life of unsealed source?
Takes into account both physical and biological half life
1/te = 1/ t1/2 + 1/ tb
What system do you use for calculating doses for unsealed sources? What is it limited by?
Medical Internal radiation dose (MIRD) system
Limited as assumes radioactivity uniform through an organ and assumes organ sizes and shapes same as standard
S values tabulated for variety of radionuclides and different source target combinations
Unclear if it is clinically beneficial
How do you estimate activity when giving unsealed source?
Simple geiger counters
- issues with dead time, rate may be higher than saying
- use geiger counter for whole body measurement and calculate immediately after given and before 1st void -> gives calibration factor - count /known administered activity. Used for subsequent counts
PET CT scans
Can use the first dose to base dosimetry of next doses on if multiple doses e.g. for NET, or can do a practice run with a small dose if only one dose being given.
Note in reality often don’t do personalised dosimetry, just give a standard dose.
When woudl you give a patient radio-iodine?
Well diff thyroid cancer (papillary and follicular)
- adj tx post total thyroidectomy if tumour >4cm, gross extra-thyroidal extension or distand mets
Dose 1.1GBq, 3.7GBq, 7.4GBq
What would you advise somebody before radio-iodine?
Low iodine diet for 1-2 weeks prior
No contrast
Stop amiodarone at least 12 weeks before
TSH stimulation x 2 IM injections
- TSH >30
What are the side effects of radio-iodine?
Acute - sialadenitis, altered taste, nausea, neck discomfort/swelling
Chronic - xerostomia, altered taste, sialadenitis/lacrimal dysfunction, increased risk of second malignany, radiation induced pulmonary fibrosis
What do you advise people after radio-iodine and how do you follow them up?
Reduce prolonged contact with friends/family Avoid public spaces SLeep away from partner Double flush toilet Wash clothes seperately
TFTs - suppress TSH for 5 yrs - depends on stage
THyroglobulin levels
9-12 month consider stimulated TSH + US neck
How does radium-223 work?
How do you monitor it?
66 cycles 55kBQ/kg 4 weeks apart Alpha emitter, high LET, <1mm DNA DSBs Mimics calcium and forms complexes with bone mineral hydroxyapatite at sites of increased bone turnover Excreted in gut
For bony mets in prostate cancer
ALP is good marker
LDH sometimes useful
PSA may not respond
Side effects: diarrhoea, myelosuppression
What are the principles of radiation protection?
Distance - inverse square law
Shielding - linear attenuation coefficient
Time - half life
Justification- must be net benefit
Optimisation- ALARP
Dose limitation- only applied to occupational exposures
What is a sievert used to measure?
Biological dose - accounts for energy absorbed and LET
1Sv = 1Gy of beta/gamma
20Sv = 1 Gy of alpha
What is a deterministic effect?
Radiation induced cell death
DOes not occur below a tissue- specific threshold dose
Severity of effect increases with dose
eg death - whole body dose =5Gy
Tends to occur due to direct DNA damage from ionising radiation
ONce threshold dose exceed the side effects go from mild to severe
What is a stochastic effect?
DNA damage by radiation
No threshold dose assumed to exist
Probability of effect increases linearly with dose
Tends to occur due to indirect effects of ioniding radiation resultsing in free radicals.
Occurs at random
Can occur many years later eg further cancer
USE ALARP
What is ALARP?
As low as reasonably practicable
Use to try to prevent stochastic effects
What is equivalent dose?
Measured in sieverts
Absorbed dose x radiation weighting factor
Weighting factor: Photons =1 Alpha = 20 Electrons = 1 Neutrons = 5-20
What is the effectiver dose?
Measured in sieverts
Equivalent dose x tissue weighting factor
Lungs, colon, breast. bone marrow, stomach = 0.12
Gonads =0.08
Bladder, liver, oesophagus, thyroid = 0.04
Bone, salivary glands, skin = 0.01
What is the average amount of background radiation per year in UK
2.2mSv
6 mSv in Cornwall
What is the average risk of an adult of developing a fatal cancer per Sv of whole body dose?
5%
What are the foetal effects of radiation?
Depends on foetal age
Threshold dose of 100-200mGy or higher
1Gy can result in mental retardation/microcephaly - particularly at 8-15 weeks
Increased risk of leukaemia
If a patient is found to be pregnant after radiotherapy has been given what should happen?
SERIOUS INCIDENT PROCESS STARTED:
- 1st priority - make estimate of likely radiation dose to foetus, establish likely date of conception, estimate distance from radiation field to edge of foetus
- 2nd priority- decide whether the incident needs to be reported under IRMER guidance.
- currently guidance is anything >10mGy should be reported
-3rd priority - carry out investigation into root cause and what can be learnt
What does IRR cover?
Covers use of all ionising radiation (allows medical exposures covered in IRMER2017). Discusses protection of staff.
What does IRR99 regulate?
Who enforces it?
Health and safety of the workplace so they are aimed at protecting people who are working with radiation
Regulation enforced by HSE
Employer must take all necessary steps to restrict as FAR AS REASONABLE PRACTICABLE exposure to radiation
Incorperates dose limits.
Does NOT cover patients - covered by IRMER
What are the 3 levels of intervention for restricting exposure in IRR99?
- Use engineering controls (prevent people from irradiating themselves)
- Use systems of work (tell people not to irradiate themselves)
- Use personal protective equipment (reduce any exposure)
WHo does the IRR appoint to manage radiation risks in a trust and what are their roles?
Radiation Protection Advisor (RPA)- roles:
- designation of controlled areas
- writing and reviewing local rules
- ensure adequate training
- ensure RPSs appointed
- monitor staff doses
- assessment of clinical incidents
- help design new faciclities
- advice to ethics committee
Radiation protection supervisor( RPS)
- supervise safe use of radiation
- inform RPA of potential hazards/incidents
- ensure safe working practice
- keep records up to date
What is a classified work? What can they do?
A worker likely to recieved >3/10 of a dose limit Dose likely to exceed 6mSV/yr Must be monitored Undergo annual medical surveillance May work in controlled areas
What is an occupationally exposed work?
UNlikely to recieved 3/10ths of a dose limit
Can work in a controlled area under a written system of work
May be monitored (not mandatory)
Must be radiation protection trained
What is the whole body annual dose limit for:
- classified work
- occupationally exposed work
- general public?
20mSV
6mSV
1mSV
What is a controlled area?
Places where it is likely the dose limnit will be exceeded
Dose rate >7.5microSv/hr
Clearly defined and marked
Can be permanent or temporary
Entry to non classified workers under written system of work
What is IRMER 17?
Governs the radiation doses received by patients for diagnosis or treatment are regulated by this legislation
What are ARSAC licences and who needs them?
Administration of Radioactive Substances Advisory Committee
Practioner licence
- use radioactive substances in medicine, the practioner has to provide evidence of training
- for any nuclear medicine, radionuclide or brachy tests
- each licence applies for one person at that site only and lists procedures the referrer can perform
- licence lasts 5 years
Employer licence
- states what facilities and support available
- governance arrangements for IRMER
- Declare procedures they plan to carry out
Under IRMER 17 who are the duty holders?
Employer - put in place standard operating procedures
Referrer- provide sufficient clinical information to allow exposure to be justified
Practitioner- justify each individual exposure
Operator- must optimise practical aspects of exposure
What does reg 10 under IRMER 17 say duty of referrer is? Can the referrer and practioner by the same?
Referrer:
- provide info to allow practitioner to justify
- must be in writing
- referrer must be identifiable
- can be referrer and practitioner but must refer to yourself in writing
What are the employers responsibilities under IRMER17?
Establish written procedures for every practive using ionising radiation
Provide ongoing staff training
Establish dose constraints for research programme
Keep diagnostic reference levels under review
Make arrangements for clinical audit
Have system for reporting unintended exposures
Which accidental exposures need to be reported?
Wrong patient exposed
Wrong radioactive material adminstered
Unintended planning or verification exposures
Dose delivered to PTV/OAR is 1.1x (10%) over whole course or 1.2 x (20%) for any fraction over
All total geographical misses
All partial geographical misses that exceed locally defined error margin
Clinically significant underdoses
Who do you report accidental exposures to?
Report to IRR99 if caused by equipment failure -> report to HSE
Report to IRMER for anything else -> CQC
How is the room a linac is in designed to reduce risk from radiation?
Thick concrete walls - prevent escape
THicker concrete walls in direct area of beam = Primary barrier
Maze - prevent radiation escaping or interlocking doors
Light gate - light sensors that cut off beam
Yellow light - controlled area
Emergency stop buttons
Beam on light
Last man out button
What is quality assurance in radiotherapy?
All procedures that ensure the consistency of medical prescription, safe fufilment of that prescription i.e. optimal treatment of PTV, sparing normal tissues with minimal exposure to staff
What is quality control in radiotherapy?
QC procedures to ensure all processes conform to established specifications i.e. checks to establish equipment is working correctly and in tolerance
quick and simple to perform
What CT QC is carried out?
Image quality
Image scaling
Laser alignment
CT number
What is the radiation calibration chain?
Primary dosimeter- held by NPL
- calorimeter for MV, electrons
- - free air ionisation chamber for kV photons
Secondary dosimeter - local, one for each type of dosimeter
- well type for brachy
- farmer for photon
- parallel plate for electron
Tertiary dosimeter - field dosimeters - used in practice
- same as secondary but multiple of them - reference dosimetry
Quaternary dosimeter
- relative dosimetry e.g. TLDs, diodes
How often is secondary dosimeter calibrated against primary dosimeter?
Every 3 years
How often are the tertiary dosimeters/field dosimeters calibrated against the secondary dosimeter?
1 yearly
What are constancy checks?
When secondary dosimeter recieved back from NPL a constancy check is performed - expose to radioactive source (strontium) and can easily correct for known decay.
This provides a baseline and is then done yearly before secondary dosimeter is used to calibrate tertiary dosimeters (Just in case dosimeter damaged over the year or something)
Also done every 3 months for the tertiary/field chambers
What are examples of linac daily QC
Safety checks/interlocks ODI SSD measurement (2mm tolerance) Output constancy checks (2%) OPtical field size of collimator and MLC (1mm/jaw) Alignment of lasers (1mm)
What are examples of linac monthly QC?
More extensive safety/interlock tests Accuracy of mechanical scales for all degrees of freedom (gantry, collimator rotation, couch) Optical cross hair vs mechanical isocentre Optical field size test Dosimetry: output, beam energy Flatness and symmetry of beam Optical vs radiation field size MLC leaf and position Emergency off switches
What is the picket field test?
MLC QA
Checks leaf positions
When prescribing radiotherapy every centre should have a standard protocol which should include….
- imaging equipment and methods
- tx techniques
- freq and timing of imaging (for verification)
- anatomical ref points
- tolerances and action levels
It is possible to prescribe off protocol but must justify
What is the record and verify system on a linac?
Machine set up by operators and compared to stored data - if does not agree than dose cannot go ahead.
What is transit dosimetry?
Combines positional and dosimetric verification
EPID = electronic portal imaging device
Exit megavoltage fluence from patient is measured by EPID
What is the dmax for 6mv, 10mv, 15mv, 20mv, 25mv?
6MV 1.5cm 10MV 2.5cm 15MV 3cm 20MV 3.5cm 25MV 5cm
Give example of the weekly QC tests for a linac?
Output calibration measurement
ODI at different distances
Pointers
Wedge factor
What is the ICRU definition of a clinically significant hotspot?
Volume outside the PTV which receives dose larger than 100% of the specified PTV dose. In general considered significant only if minimum diameter exceeds 15 mm; however, if it occurs in a small organ (e.g. the eye, optic nerve, larynx), a dimension smaller than 15 mm has to be considered
What is acceptable dose hetereogeneity of the prescribed dose?
+5 to -7%
What is the conformity index?
RTOG conformity index (ideally 1 usually 1.05-1.3) = Treated volume / PTV
Paddick conformity index (ideally 1 usually 0.7-0.98) = PTV that is irradiated / irradiated treatment volume X PTV that is irradiated / PTV
Paddick conformity index is more useful as actually makes sure the irradiated volume overlaps with the PTV.
Gradient index (want as small as possible usually 3-7, is a measure of dose gradient) = volume of patient receiving 50% of prescription isodose = volume of patient receiving prescription dose
In brachytherapy (Paris system) how is the mean basal dose rate determined? How is the reference dose related to the mean basal dose rate?
Mean basal dose rate is the average of the minimum dose rates located between the sources inside the implanted volume. The individual minimum dose rates should be within ±10% of the average (basal dose rate), thus restricting the number of sources to be used The stated (reference) dose rate is a fixed percentage (85%) of the basal dose rate.
What is a free radical?
A free radical is an atom or molecule with an unpaired electron, regardless of its charge status, making it highly reactive with other atoms and molecules. Free radicals have halflives on the order of micro- to milliseconds. Free radicals do not necessarily possess charge.
Describe the 3D cervix brachytherapy planning as per GEC-ESTRO
Define GTV, high risk CTV (high tumour volume), intermediate risk CTV (microscopic spread) and OARs
Has recommendations on dose prescriptions and OAR limits (D2cc = the minimum dose to the most irradiated 2 cubic cm volume of OAR)
DVH reporting for rectum, bladder, sigmoid and small bowel
Defined reporting points/volumes: point A, total reference air KERMA for whole treatment, high risk CTV and intermediate risk CTV (D98%, D90% and D50%), GTVres (D98%) = residual GTV following external radiotherapy
Defined OAR volumes: bladder/rectum/sigmoid - D0.1cc, D2cc, ICRU38points); vaginal wall using surrogate posterior inferior border symphysis +/- 2cm
EQD2 used to sum the EBRT and brachy in 2 Gy/fraction equivalence
Use MR based brachytherapy planning for soft tissue (CT often used for applicator reconstruction then CT and MR registered)
Describe the manchester intracavitary system
Use intrauterine applicator and two vaginal ovoids.
Prescribe dose to Point A (2cm superior to surface of cervical OS/ovoid and 2cm lateral to midline/IU tube) - this represents the minimum dose to malignant tissue. Point A should not receive more than 1/3 dose from vaginal ovoids - creates pear-shaped dose distribution.
Calculate dose to Point B (3cm lateral to A - dose estimate to pevlic wall)
Calculate dose to rectal (0.5cm posterior to vaginal wall through plane of vaginal ovoids) and bladder dose points (surface of bladder balloon).
Evolved with better imaging, and variation in source dwell times. Then could add interstitial brachytherapy to bring dose out for irregular tumours but need TPS planning by GEC estro guidelines.
Describe prostate brachytherapy
Online planning using transrectal ultrasound (patient under GA and lithotomy position) with grid against perineum in which needles inserted. Contoured prostate and rectum and urethra and CTV. Insert I125 seeds through needles. GEC ESTRO define dose rules.
HDR afterloader based system can use offline planning with CT after needles inserted then insert sources after planning on CT.
What are the principles of beam matching?
Isocentre located at point where PTVs split. Need to ensure no divergence/overlap at the junction. e.g. can get hotspot if electrons next to photons as electrons will diverge into photon field.
Main example: lateral beams in H+N above neck then A+P below neck.
What are the definitions of beam symmetry and beam flatness?
Flatness = Dmax/Dmin
Symmetry = max (D -x / D +x)
Symmetry is the difference between the doses at equidistant points along the beam profile from the midline.
