small field dosimetry Flashcards
what is AAPM TG report for smal field dosimetry and other important reports
TG 155
TRS 483
main issue with small fields
lack of lateral charged particle equilibrium
high Z detectors are used in small dosimeters to amplify signal. What is issue with always using these?
Also, how does silicon in detectors impact small fields?
High Z detectors have high energy dependence due to PE effect. This is worse for larger fields as there is more low energy scatter (higher PE). It also changes with depth due to more low energy scatter with depth.
Also relevant for small fields because output factors are normalized to those in larger reference fields (hence why we use daisy chaining)
silicon introduces over-reponse in small fields due to a fluence pertubation!
-daisy chain technique helps to account for energy dependence at large fields, but doesn’t account for pertubation effects at small fields
what are issues with large ion chambers in small fields?
volume averaging effect
what is issue with small field rgarding position of detector?
sensitive part of detector has to be in center of small field- can be difficult to do
how small should the ion chamber be?
ion chamber diameter must be < 1/3 of field diameter
what is AAPM report on MV photon beam dosimetry in small fields and non-equilibrium conditions?
TG155
3 challenges in small fields
- loss of lateral CPE
- occlusion of primary photon source
- limited choice of suitable detectors
where does loss of LCPE occur?
- interface of media with different densities
- penumbra region
- interior portion of narrow fields
define machine specific reference field
reference field for a machine like tomo, gamma knife, cyber (i.e no 10x10 and 100 cm SSD)
max energy to use in small fields
Per ICRU 91, 10 MV, due to larger range of secondary electrons
procedure for dosimetry of small fields
correct output factors with MC factors to go from ratio of readings to ratio of absorbed dose
- output factors are for clinical field to machine specific reference field (or other intermediate fields if daisy chaining)
- use factors to relate dose at msr vs reference then clinical vs msr
definition of small field
- loss of lateral CPE
- generally < 3x3 cm2
how does the detector material affect pertubation in small fields?
-high density detectors over respond and low density detectors under respond
should one compare results to small fields for other machines?
yes, but make sure they have same MLCs
-can also use IROC MD Anderson audit as 3rd party check
pros and cons of liquid ion chambers
lower pertubation effects due to near-water equivalence of dielectric liquids
-correcting for ion recombination is challenging as cannot use standard methods- collected charge does not increase linearly with voltage and only saturates at voltages too high that are suitable for the insulators used in the detector
do diamond detectors cause pertubations?
yes
-similar Z to water but different density
what is a passive detector?
has to be read out after measurement (ex. film)
can TLDs be used for small fields?
yes, down to 0.6x0.6 cm2 and with correction factor
-higher density of TLDs and OSLDs causes electron fluence pertubation in small fields
describe polymer gel
respond to radiation with polymerization reaction, forms polymer microparticles proportional to dose
distribution of particles can be imaged with MRI, optical CT, or x-ray CT
-don’t have issues with directional dependency or volume averaging
most suitable detectors for PDDs with small fields
micro ion chambers and PP chambers, senstive volume , 0.02 cm3
-have to consider that pertubation effects vary with depth- cannot just take ratios for PDD like one would with a larger field
what are pertubation effects?
departures from ideal BG or SA cavity theory
msr of tomo unit
5x10 cm at 85 cm SSD
cyber knife msr
diameter of 6 cm, SCD of 80 cm
explain how to acquire diagonal profile
don’t rotate collimator as this would remove FF dependence
-move the tank. make sure there is enough water for scatter in the corner
report for dosimetry of small fields
IAEA technical report series no 483
AAPM report for accelrator beam data commissioning
TG106
issues with alignment of detector in small fields
more sensitive to alignment issues
visualize light field cross hair wrt detector
misalignment would cause PDD to fall off too fast
how to determine field size for small fields
measure FWHM
console field size not accurate enough
FWHM gets wider than geometric field size as field size decreases
how to find central axis of radiation field?
raster scan
also will allow you to find center of sensitive part of detector
issues with detector orientation
IC- stem effect has to be consistent
diode- hysteresis with high Z component if it goes in and out of the measured field
best orientation for diode also depends on cerenkov
Issue with reference detector
may shield signal detector
if so, remove reference detector and integrate or try to take a dose rate reading directly from MU chamber (slow down scanning rate such that dose rate is stabilized)
definitions of small field
where we are occluding the photon source (not an issue in modern linacs with small sources)
- lateral CPE is lost throughout most of field
- field small compared to detector
pros and cons of ion chambers in small fields
- kq corrects for energy
- familiar
- volume effect
pros and cons of micro ion chambers in small fields
kq corrects for energy
stem effect, signal to noise ratio is low
spatial resolution high
pros and cons of stereo diode in small fields
ageing with radiation
dose rate dependence
normalize to 4 cm2 for energy dpeendence
small size, available
pros and cons of diamond detector in small fields
almost no energy dependence
weak dose rate dependence
almost ideal
pros and cons of plastic scintillator in small fields
temperature dependence
low signal to noice
almost no energy dependence
pros and cons of gels in small fields
good spatial resolution
dependence depends on gel material
not easily available, have to scan with MRI or CT
largest detector dimension that should be used?
1/3 of the field size
what spectral changes occur with reduced field size?
decrease of head and phantom scatter reaching detector. Phnatom scatter most important effect.
what do the TRS 483 small field output factors correct for?
- volume averaging
- density of detector relative to water
- effective atomic number
- detailed detector construction/geometry
- always used FWHM for field size (not set field size)
formula for equivalent square field sizes for small fields
Sclin = sqrt(AB) Sclin = 1.77 r (circular fields)
explain process for measuring small field output factors
- Use multiple detectors, all appropriate to field dimension
- Each detector has its advantage/disadvantage for small fields
- Measure FWHM field sizes
- Apply correction factors
- Double check measurements
- Compare to literature
should you input small field info into Eclipse?
- beam configuration document suggests data won’t be used
- but varian’s own report show the effect of including smaller field size data for fields >/= 1 cm
E2E stereo dosimetry tests has what passing criteria?
1 mm for localization
5% for dose
what test must be done immediately prior to SRS, and if no SRS, then weekly?
Winston Lutz test
how do you do dose verification for stereo plans with multiple mets?
measure for largest and smallest met
name of the hidden target stereo E2E phantom?
MAX-HD
make sure phantom is compatible with image registration alogorithm
Exradin A12 is good to what dimension?
4x4 cm^2
