SU‐E‐T‐194: Accounting for Support Arm Backscatter in EPID‐Based IMRT QA

Purpose: To devise a method for IMRT QA using a Pinnacle model for the EPID response which accounts for non‐uniform backscatter from the support arm. Method: The method is based on a Pinnacle treatment planning system model. It relies on determining a correction matrix (CM) that multiplies the (flood‐field calibrated) EPID image to restore pixel response to backscatter and subsequently constructing a backscatter region within the Pinnacle model …