SU-E-T-145: Scanned Percent Depth Dose Curve Discrepancy for Varian TrueBeam Flattening-Filter-Free Photon Beams Using Very Small Sensitive Volume Ion Chamber

Abstract

Purpose: To investigate the discrepancy of scanned percent depth dose (PDD) for Varian TrueBeam flattening‐filter‐free (FFF) photon beams when using ion chambers with different sensitive volumes.Methods/Materials: PDD curves were scanned for both Flattening Filtered (FF) and FFF photon beams (6MV and 10MV) by using two common scanning chambers with different sensitive volumes — Exradin A1 (0.053cm3) and PTW23323 (0.1cm3). Both Pion and Ppol of two chambers for FFF beams were measured based on TG‐51 protocol at depth of dmax, 10cm and 20cm. The PDD curves from the two chambers were compared, as along with the Pion and Ppol at different depths. Results: Scanned PDD curves of the same energy FF beams were almost identical between two chambers, but the FFF beams showed non‐trivial differences. The differences increased with depth, and were more pronounced for 10MV beam. PDD of FFF beams scanned with different dose rates showed very little difference.For FF beams, all PDDs agreed well with each other at 10cm depth. For FFF beams, PDD of PTW23323 agreed with Varian specifications, but PDD of Exradin A1 showed significant differences at 10cm depth. The PDDs at 10 cm depth of the PTW23323 agreed with Varian specifications, golden beam data provided by RPC and the published TrueBeam commissioning data.Both chambers can be seen to yield Pion and Ppol that agree within 0.6%. Conclusion: We observed a significant difference in scanned PDD curves for FFF beams obtained when using different sensitive volume ion chambers. The PDD curves scanned with the very small sensitive volume ion chamber showed significant difference from both larger chamber results and golden beam data, beyond 10cm depth. If calibration of the FFF beam output was performed based on PDD scanned with a small sensitive volume ion chamber, the output calibration difference could reach 3%.