Program Information
Absolute Dose Verification of FFF Beams for AAA Algorithm Using CIRS Electron Density Phantom
V Mhatre*, P Patwe , Sir HN RF Hospital, Mumbai, Maharashtra
Presentations
SU-I-GPD-T-402 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: Aim of this study was to measure the absolute dose using AAA dose calculation algorithm for flattened and FFF beams for 6 MV using CIRS electron density phantom (EDP).
Methods: The CIRS EDP with various tissue equivalent plugs (lung exhale,lung inhale,liver,breast,muscle, adipose,bone)was used in this study.The absolute dose was measured using pinpoint ionization chamber. The 6 MV photon beam was investigated in this study since higher beam energy photon beams have a less inhomogeneity correction effect.Plans were created using the Varian Eclipse treatment planning system (TPS) AAA v.13.0 heterogeneity correction algorithm.Two plans were created one for 6 MV flattened & one for 6 MV FFF beams with gantry and collimator 0° on TrueBeam STx.A dose of 200 cGy was prescribed at a depth of 5.5 cm for all plans with a field size of 4x4 cm².To measure the actual dose delivered to the prescription point,the individual plans were delivered to treat the EDP as shown in fig 1.The MUs delivered were defined by the related treatment plans and the dose was measured using pinpoint ion chamber.All measured doses were compared with calculated doses from treatment plans.
Results: The absolute dose was found to be most consistent with the calculated dose for flattened beams for AAA algorithm.For lung and bone inserts the variation was at the level of 3% for FFF beams .
Conclusion: Our study indicates that the AAA algorithm for FFF beams can reach an accuracy of 2% suggested by AAPM TG 65 for all types of tissues except for lung and bone inserts considering the uncertainty of ion chamber and electrometer.
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