SU‐E‐T‐460: Impact of the LINAC Repetition Rate On a High‐Resolution Liquid Ionization Chamber Array for Patient‐Specific QA

Abstract

Purpose:The purpose of this study is to investigate the LINAC repetition‐rate (dose‐rate) dependence of OCTAVIUS 1000SRS liquid ionization chamber (LIC) array for patient specific QA of SRT plans delivered with flattening‐filter‐free (FFF) beams.Methods:1) The repetition‐rate dependence of 1000SRS was measured in a phantom constructed with 5‐cm solid water above and below the array for build‐up and backscatter. A 0.3cc calibrated ion chamber was also placed along the central axis 2.3cm below the center chamber of the array for normalizing LINAC output fluctuation. The signals from the center chamber of the array under different repetition rates in the range of 400–2400 MU/min for 6xFFF and 10xFFF beams on a Varian TrueBeamSTx LINAC, normalized by the independent chamber readings, were analyzed for the array response dependence on repetition rates. 2) Twelve Step‐and‐shoot IMRS QA plans (6xFFF and 10xFFF) were delivered to the array under different repetition rates for analysis and comparison. 3) The absolute doses measured by the center chamber were compared to measurements using an independent ionization chamber with the identical setup, taken as the gold standard. 4) The correction factors based on the actual delivery repetition rate were applied to the measurements, and the results were compared again to the gold standard.Results:1) The 1000SRS array exhibited repetition‐rate dependence for FFF beams, up to 5% for 6xFFF and 10% for 10xFFF; 2) The array showed clinically‐acceptable repetition‐rate dependence for regular flattened beams; 3) This repetition‐rate dependence significantly affected the measurement accuracy, thereby affecting IMRS QA results; 4) By applying an empirical repetition‐rate correction, the corrected measurements agreed better with the gold standard ion chamber measurements.Conclusion:OCTAVIUS 1000SRS LIC array exhibited considerable repetition‐rate dependence for FFF beams, which will affect the accuracy of the absolute QA measurements, especially for IMRS plans with the step‐and‐shoot technique.