SU‐E‐T‐476: Volumetric Modulated Arc Therapy (VMAT) Plan Validation Using MatriXX Measurements and Monte Carlo Calculation

Purpose: To establish confidence in VMAT plan delivery for recently launched VMAT with Varianˈs RapidArc technique at our clinic, a sophisticated plan validation has been performed by comparison of ion- chamber measurements, planning system calculations, and independent Monte Carlo calculations. Methods: Although MatriXX has been used as a reliable device for validations of intensity modulate radiation therapy (IMRT) treatment plans, it was challenging to use it for VMAT plan validations because of its angle-dependent dose response. A new version of MatriXX Evolution with a gantry angle sensor is able to correct the angle dependency and promises accurate measurements while the gantry rotates. To evaluate the performance of MatriXX for RapidArc plan validations, Monte Carlo calculations using XVMC was used as an independent method to our IMRT validation program. DICOM plans and CT images from Eclipse planning system were translated by an in-house code for XVMC input. Measured dose planes were compared with XVMC dose planes as well as Eclipse dose planes for 12 pelvis plans, 19 prostate plans, and 5 head and neck (H&N) plans. Construction of a scatterplot by taking differences of calculations from measurements displayed a positive correlation between them. Results: Eclipse calculations were off from measurements by 2.64%, 1.89%, and 1.84% in quadratic mean for pelvis, prostate, and H&N plans, respectively. XVMC calculations were off from the measurement by 2.18%, 1.73%, and 2.30% in quadric mean for pelvis, prostate, and HN plans, respectively. Passing percent of Gamma test for Eclipse calculations was over 97.0% and that for XVMC calculations was about 95.7% due to inherent statistical uncertainty of Monte Carlo calculation. Eclipse and XVMC calculations agreed to each other within 4 % Conclusions: The present study shows that Eclipse calculations and measurements using MatriXX evolution can be used for validation of VMAT plans using Rapid Arc delivery technique.