Sci—Thur AM: YIS ‐ 05: 10X‐FFF VMAT for Lung SABR: an Investigation of Peripheral Dose

Abstract

Flattening Filter Free (FFF) beams exhibit high dose rates, reduced head scatter, leaf transmission and leakage radiation. For VMAT lung SABR, treatment time can be significantly reduced using high dose rate FFF beams while maintaining plan quality and accuracy. Another possible advantage offered by FFF beams for VMAT lung SABR is the reduction in peripheral dose. The focus of this study was to investigate and quantify the reduction of peripheral dose offered by FFF beams for VMAT lung SABR.The peripheral doses delivered by VMAT Lung SABR treatments using FFF and flattened beams were investigated for the Varian Truebeam linac. This study was conducted in three stages, (1): ion chamber measurement of peripheral dose for various plans, (2): validation of AAA, Acuros XB and Monte Carlo for peripheral dose using measured data, and (3): using the validated Monte Carlo model to evaluate peripheral doses for 6 VMAT lung SABR treatments. Three energies, 6X, 10X, and 10X‐FFF were used for all stages.Measured data indicates that 10X‐FFF delivers the lowest peripheral dose of the three energies studied. AAA and Acuros XB dose calculation algorithms were identified as inadequate, and Monte Carlo was validated for accurate peripheral dose prediction. The Monte Carlo‐calculated VMAT lung SABR plans show a significant reduction in peripheral dose for 10X‐FFF plans compared to the standard 6X plans, while no significant reduction was showed when compared to 10X. This reduction combined with shorter treatment time makes 10X‐FFF beams the optimal choice for superior VMAT lung SABR treatments.