TG120 Flashcards
What is TG120 on?
dosimetry tools and techniques for IMRT
different between IMRT QA vs 3DCRT
-3D complexity of dose, decoupling of beam geometry and resulting dose
-IMRT has to focus on cumulative dose rather than QA of inidividual segments, and has to check dose at different locations
-cannot use portal images to check OAR avoidance like you can with 3D
-3D can be checked with calculations- with IMRT it is harder for a physicist to see an issue
2 ways volume averaging is a problem
-small field- measure dose will be smaller than actual
-profiles of fast-varying doses- dose distribution will be blurred
-however, if volume averaging is NOT an issue, use larger ion chamber as it will be less sensitive to positioning errors
tests in acceptance testing of ion chambers
-radiographs to verify mechanically sound
-leakage current < 10^-14 A
-quantify stem effect
-test microphonics (currents generated by flexing cables)
-radiation equilibration time - should be stable after 2 readings of 200 cGy
-test atmospheric communication
-polarity effects < 1 %, realistically < 0.3%
-collection efficnecy < 1.02
-test orientation dependence
shielded diodes
-shield is meant to compensate for diode sensitivity to low E photons
-shielded diodes are ok on axis, but not off axis
sensitivity of diode compared to ion chamber
20-100X
smallest ion chamber that can be used
FS must be 1.5 cm larger than effective length of ion chamber
for IMRT dose measurements, what size ion chamber should you use?
size should be small enough to limit the dose heterogeneity across the chamber active volume to 10% and 5% if the measurements arebeing compared against volume-averaged and point doses, respectively
what detector to use for measuring MLC penumbra used in TPS modelling
diode
-diode is good for measuring relative dose distribution
where should diode detectors not be used?
-absolute doses
-as sole meaurement device to model IMRT beam penumbra, b/c of poential for over-response in low dose region. Compar with other detectors with little energy response
where should diode be used
-measuring relative dose distributions,particularly for measuring MLC penumbrasused in beam modeling in a treatment plan-ning system
-dose at points supplemental to ion chambers
when should TLDs be used
When the phantom geometry will not allowi onization chamber measurements.
When multiple simultaneous point measure-ments are desired.
when should TLDs not be used
-if measurement precision needs to be better than 3 %
what TLD should be used
low Z
LiF
when should ion chambers be used
-In homogeneous dose regions
-To verify monitor unit outputs
-To verify critical structure doses
when should ion chambers not be used
-To measure beam profiles that will be em-ployed to model the IMRT beam penumbra.
-When the leakage current will yield an inte-grated charge of >5% of the expected radiation-induced charge. Leakage current corrections should be applied if the expected uncorrected error is>2%.
when should radiographic film be used?
-For relative IMRT dose distribution measurements
-To measure beam profiles that will be employed to model the IMRT beam penumbra
-For measuring relative output factors of small fields
when should radiographic film not be used?
-absolute dose measurement
-verify MU
when should radiochromic film be used?
-For measuring relative dose distribu-tions
-For measuring dose distributions that will be used to model the IMRT beam penumbra
-For measuring relative output factors of small fields
-When a radiographic film processor is not available
when should radiochromic film not be used?
-absolute dose measurement
-verify MU
where should 2D arrays be used?
-routine QA of a precommissioned IMRT technique- initial commissioning should be done with film
issue with using CR for IMRT QA
-Photon spectrum varies widely across IMRT fields
-some use low-energy filters
what does phantom characterization entail?
-verify dimensions before first use
-look for any voids or flaws in phantom (CT)
where is DTA sensitive
-sensitive in shallow dose gradient region but insensitive to even large dose differences in steep dose gradient regions
-dose difference tool is overly sensitive in steep dose gradient regions
composite tool
regions that fail both DTA and dose difference
-pass/fail- doesn’t give a magnitude
where is gamma evaluation useful?
-large amount of dose data needs to be reviewed quickly, such as for routine patient QA.When discrepancies are identified, the clinical impact of those discrepancies can be determined using the dose-difference tool
does EPID or 2D array have finer resolution?
EPID unless array is high res like Octavius…
what does truly comprehensive IMRT verification require?
3D detector
ex. gel
