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
