portal dosimetry

Question 1

can you for portal dosimetry at any SSD?

Answer 1

Yes, but system will auto distance-scale the resultant dose image to SSD 100 and will not apply IS to scale the dose

Question 2

explain gamma 3D test

Answer 2

Our gamma is 3 %/3 mm, normalize to dmax, only looks at points within 10 % of dmax
– Gamma- looks at distance to agreement and difference in dose value
-calculates (actual distance to agreement/ 3 mm)^2
-calculates ((dose error/Dmax)/3%)^2
gamma value is square root of sum of the two above

Question 3

Pros and cons of different detectors wrt absolute dose

Answer 3

vented ion chamber- can be calibrated and measure absolute dose
diodes- need cross calibration
liquid filled IC- need cross calibration

Question 4

pros and cons of different detectors wrt detector size and resolution

Answer 4

vented IC: large
liquid filled IC: smaller
diodes: smallest

Question 5

pros and cons of different detectors wrt angular dependence

Answer 5

almost all detectors experience angular dependence

• use inclinometer and internal corrections
• make phantoms that account for the issue
• irradiate only perpendicular to the detector

Question 6

pros and cons of different detectors wrt energy dependence

Answer 6

• diodes have energy dependence- need separate calibration files
• ionization chambers have minimal E dependence

Question 7

different detectors that do 4D

Answer 7

• Octavius 4D- flat panel inside a cylinder and panel rotates
• ark check- diodes lie along the cylinder
• delta 4- x shaped panels- don’t rotate
• arck check and delta 4 may have worse resolution but you can rotate arc check by a few mm and retake measurements to get better resolution

Question 8

detector type in arc check, octavius, and delta 4

Answer 8

arc check and delta 4 are diodes

octavius is liquid filled PP IC (for SRS) or vented IC

Question 9

resolution of arc check, octavius, delta 4, matrixx

Answer 9

mapcheck and matrixx ~ 7 mm
octavius vented IC is 10 mm
octavius liquid filled PP IC is 2.5/5 mm
delta4 is 5/10 mm

Question 10

do plane parrelel liquid filled ion chambers (Octavius) have dose rate dependence?

Answer 10

yes

Question 11

type of ion chambers in profiler

Answer 11

parrallel -plate

Question 12

IMRT/VMAT QA process

Answer 12

• plan is calcualted on phantom geometry

- calculated dose for a known plane location is exported