Equipment Flashcards
Where is FFF used?
Tomotherapy is 6MV FFF
Cyberknife uses 6MV FFF
Standard linacs can now use FFF
Why use a flattening filter?
Bremmstrahhlung distribution is strongly forward peaked in MV range, get bell shaped distribution, FF attenuates centre of beam giving flat distribution
Disadvantages of flattening filter (5)
Significant loss of dose rate due to attenuation (1/2-1/4)
Major source of head scatter increasing scatter dose to patient
Lateral spectral changes make modelling difficult
Accentuates collimator exchange effect
Acts to amplify beam steering or energy errors
Advantages of FFF (6)
Inverse planning means flat beam no longer necessary
Increase in dose rate
Reduction in extra focal scatter, less scatter to patient
Easier to model in TPS
Reduction in collimator exchange effect becomes negligable
Beam less prone to steering induced assymetries
What is varian’s approach to FFF?
Removed flattening filter and preserved beam running parameters (RF, gun, bending system parameters)
What is Elekta’s approach to FFF?
Remove FF and adjust running parameters
Dose at 10cm would reduce from 67.5 to 65%, adjusted to bring it back to 67.5
How do FFF PDDs vary in elekta and varian?
Elekta, FF and FFF very similar
Varian, FFF less penetrating
What are considerations in FFF reference dosimetry (correction factors)?
Chamber sensitivity - function of beam spectra kfff
Higher dose rate - large increase in dose per pulse and therefore ion recombination kion
Non-uniformity - shape of beam across chamber no longer negligible kvol
Chamber sensitivity kfff
With FFF, same TPR have different energy spectra, so correction is required. NPL determine a beam quality correction factor
Chamber sensitivity is a function of beam spectra, which has changed
Ion recombination kion
Factor accounts for incomplete collection of ion pairs in ion chamber. Function of charge intensity. Dose rate higher, dose per pulse higher, and therefore charge intensity. Need to correct for this.
Beam uniformity kvol
Beam profile reduces chamber response for 2 reasons: volume averaging (0.1%) and perturbation of secondary electron fluence by air cavity.
NPL recommend beam uniformity correction of 1 for NE 2611 chamber. None recommended for field chamber, incorporated into calibration factor
Substitution rather than simultaneous irradiation
Commissioning considerations FFF
No additional measurements but need to consider FFF beam when taking existing measurements: what detector is appropriate?
Ensure detector etc can measure increased dose rate
Spatial resolution, dose gradients in more directions, size vs signal/noise
Spectral sensitivity of detector, diode detectors can be more sensitive to spectral changes than chambers
Ion recombination losses - higher and can vary between chambers
Treatment times with FFF
Can drop dramatically due to dose rate. Can make breath hold more possible for some breast patients. Can use MLC, very homogenous, no need for wedge and field in field
Can be used in lung SABR and hypo-fractionated prostates to give speedier delivery
What makes a field small?
Lateral charged particle equilibrium not achieved on beam axis
Primary source is partially occluded
Detector is similar to or larger than the beam dimensions
When are small field TPS calculations used?
Stereotactic treatment
Small conformal fields
Small or narrow segments making up larger fields in IMRT or VMAT
LCPE loss
Beam radius < max range of secondary electrons
Field size for loss of LCPE is energy dependent: greater for higher energy
How does head scatter change with field size and why?
Head scatter drops sharply at small fields, amount varies with linac
Not related to FF, function of extended source occlusion
Why does source occlusion occur?
The source of photons is point where electron beam hits target, and is gaussian shaped
Beam is composite of direct and scattered radiation
The output drops significantly when direct radiation is blocked
What is typical source size?
2mm - 5mm
What effects do TPS need to model with source occlusion?
FWHM no longer correlates with field size
Needs to model source size accurately to correctly predict CAX dose
What is standard definition of the field size?
FWHM
When does FWHM stop correlating with physical collimator opening and how is this wrong?
When penumbrae start to overlap with CAX
Distance between individual jaw 50% is less than width of 50% of profile (difference depends on energy and collimator design)
What might need doing if accuracy of field width at small and large fields isn’t acceptable?
Commission two separate linacs in TPS
What are compound collimators?
Combination of collimators, solid jaws and MLC, differ between linacs
What happens if TPS thinks source size is larger than in reality?
Will incorrectly assume source shielding and expected output will fall
How does FF affect beam in a small field?
View of FF restricted so source of scatter reduces
Energy spectrum changes
What affect does FF have on small vs large field size?
Large fields in flattened beams have more low energy photons
Calculation matrix in small fields
Too course a grid shows errors up to 45%, grid spacing for 10mm segments should be 1mm or smaller
Why can’t detectors in large fields be assumed to work in small field?
Changed spectrum
Pertubation of chamber
Volume averaging
How far from detector edge be from field edge?
r_LCPE F
What issues are associated with small chambers?
Small ionisation chambers have small chamber signals, means signal generated in cable becomes more significant
How are smaller detectors often made?
Using a interacting material of increased density
What are impacts of using a higher density detector?
Electron range is lower
Electrons can exit cavity less easily
Dose higher than in water cavity
Measured dose does not represent dose in absence of detector
What do we need to consider when measuring profiles and how woud we check the profile?
Select detector with sufficient resolving power
Check the profile by measuring with multiple detectors and finding point where profile shape does not change
Measurement of PDDs of small fields
Check source alignment with collimator axis, danger of moving off beam axis with changing depth
Measure on beam axis (find max with serpentine scanning)
Corroborate with multiple methods and detectors
Correct energy dependent detectors for increase in low energy scattered photons component with depth
Detector for PDD
Mini ionisation chamber
Diodes
Radiochromic film (uniformity correction and calibration must be good)
Diamond with detector specific dose rate correction
TPR in small field
Usually calculated from PDD but in small field lack of LCPE means can’t assume published methods are valid
Recommended to take direct measurements
Full scatter factor calculation recommendations
Detector must be smaller than smallest field
Could use small LIC, radiochromic film, diodes (with daisy-chain)
Measurement, scan to find point of maximum dose foe reach field, use multiple detectors
Calculation of head scatter factor in small field
Need complete phantom in smallest field. LCPE required
High density caps used, higher density mini phantom with smaller detector. Cannot use in large fields due to mix of high and low energy photons
Validating Sc in small fields
Overlap field sizes for small and standard measurement systems
Measure with multiple methods or detectors
Extrapolate to zero area FS, should cross axis close to zero
Compare with measurements made on matched beams
Phantom scatter factor in small fields
Normally Scp / Sp
Density scaling method looks promising. Should be able to use published Sp data if beam spectrum matched
What is density scaling theorum?
If FS and depth are scaled inversely with density, scatter to primary ratio (SPR) is constant. Measure in low density medium and calculate value in unit density medium using density scaling.
Works if depth is constant but field size is changed: Sp for 3x3cm beam in unit density material can be measured in 6x6cm beam in material halfthat density.
3 possible sources of error in TPS for dose calculations in small fields
Source size - is occlusion occurring
FWHM no longer corresponds to field size
Calculation matrix - if too large then can have large impact on dose estimation
Differences in beam characteristics of FFF vs cFF
Dose rate
CAX depth dose (PDD)
Profile
Scatter factors/field size factors
Out of field dose
Define beam flatness
Flatness is the range of maximum to minimum relative dose across a determined central section
of a beam profile
Define beam symmetry
Symmetry is the ratio of relative dose at points equidistant from the central axis across a
determined central section of a beam profile
What differences are irreconcilable in FFF?
Different beam spectra on central axis and laterally
Changes in build up region
Why are there changes in the build up region in FFF?
Electron contamination differs, affecting build up and dmax
Which treatments can benefit from FFF?
Breast with breath hold - more patients can hold breath for less time
SABR - speedier delivery is better as accuracy decays with time, want to delivery high dose accurately
How does output factor vary for FFF?
Due to the reduction in scatter, the variation of total output factor with field size is much less pronounced than that for a flattened field.
What is difference in head in FFF?
Uniform 2mm stainless steel filter replaced blanking filter in carousel - prevents electron beam reaching patient if target fails, produces electron fluence at level of ion chamber.
