patient dosimetry

Question 1

issue with high Z detectors

Answer 1

over-response for low energy scattered photons

-issue in larger fields, heavily blocked fields

Question 2

considerations for IMRT dosimetry

Answer 2

• 3D CRT- can measure a subset of beams is sufficient
• IMRT: fluence is varied during treatment so requires integrative dosimetric techniques
• IMRT- sometimes independent dose calculation methods are used as QA
• have to consider oblique-incidence response of clyndrical ion chambers for non-coplanar beams
• Volume averaging is an issue for detectors in small fields- dose will be underestimated due to averaging over the entire size of the detector
• if measuring profiles of dose distributions with rapidly spatially varying doses, the shape of the dose distribution will be blurred by volume averaging – use higher spatial resolution detector if accurate measurement of profile shape is needed

Question 3

what is arc check?

Answer 3

4D array which “measures and correlates gantry angle, leaf-end position, absolute dose and time to identify any potential source of error throughout the patient volume