TG120

Question 1

What is TG120 on?

Answer 1

dosimetry tools and techniques for IMRT

Question 2

different between IMRT QA vs 3DCRT

Answer 2

-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

Question 3

2 ways volume averaging is a problem

Answer 3

-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

Question 4

tests in acceptance testing of ion chambers

Answer 4

-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

Question 5

shielded diodes

Answer 5

-shield is meant to compensate for diode sensitivity to low E photons
-shielded diodes are ok on axis, but not off axis

Question 6

sensitivity of diode compared to ion chamber

Answer 6

20-100X

Question 7

smallest ion chamber that can be used

Answer 7

FS must be 1.5 cm larger than effective length of ion chamber

Question 8

for IMRT dose measurements, what size ion chamber should you use?

Answer 8

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

Question 9

what detector to use for measuring MLC penumbra used in TPS modelling

Answer 9

diode
-diode is good for measuring relative dose distribution

Question 10

where should diode detectors not be used?

Answer 10

-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

Question 11

where should diode be used

Answer 11

-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

Question 12

when should TLDs be used

Answer 12

When the phantom geometry will not allowi onization chamber measurements.

When multiple simultaneous point measure-ments are desired.

Question 13

when should TLDs not be used

Answer 13

-if measurement precision needs to be better than 3 %

Question 14

what TLD should be used

Answer 14

low Z
LiF

Question 15

when should ion chambers be used

Answer 15

-In homogeneous dose regions
-To verify monitor unit outputs
-To verify critical structure doses

Question 16

when should ion chambers not be used

Answer 16

-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%.

Question 17

when should radiographic film be used?

Answer 17

-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

Question 18

when should radiographic film not be used?

Answer 18

-absolute dose measurement
-verify MU

Question 19

when should radiochromic film be used?

Answer 19

-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

Question 20

when should radiochromic film not be used?

Answer 20

-absolute dose measurement
-verify MU

Question 21

where should 2D arrays be used?

Answer 21

-routine QA of a precommissioned IMRT technique- initial commissioning should be done with film

Question 22

issue with using CR for IMRT QA

Answer 22

-Photon spectrum varies widely across IMRT fields
-some use low-energy filters

Question 23

what does phantom characterization entail?

Answer 23

-verify dimensions before first use
-look for any voids or flaws in phantom (CT)

Question 24

where is DTA sensitive

Answer 24

-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

Question 25

composite tool

Answer 25

regions that fail both DTA and dose difference
-pass/fail- doesn’t give a magnitude

Question 26

where is gamma evaluation useful?

Answer 26

-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

Question 27

does EPID or 2D array have finer resolution?

Answer 27

EPID unless array is high res like Octavius…

Question 28

what does truly comprehensive IMRT verification require?

Answer 28

3D detector
ex. gel