SU-E-T-522: Modeling the Agility MLC for Monte Carlo IMRT and VMAT Calculations

Abstract

Purpose: The Agility multileaf collimator (MLC) mounted on an Elekta Synergy linear accelerator for 6 MV was modeled for IMRT and VMAT calculations using the BEAMnrc Monte Carlo (MC) code and verified versus measurements. Methods: To describe the Agility MLC in BEAMnrc, the available Component Module code was modified to include its characteristics; 5 mm leaf width, flat leaf sides with a focus point shifted from the radiation source. The MLC model was verified by comparison of the calculated interleaf leakage and tongue‐and‐groove effect for a closed MLC field and an irregular field to measurements with EBT2 film in a solid water phantom and diode measurements in a water phantom, respectively. We have developed a time dependent phase space data (PSD), which include a parameter based on MU index. Because leaf, jaw, collimator and gantry positions of each segment are controlled by MU index, this PSD enabled to simulate dynamic motions by interpolating positions between each segment. IMRT and VMAT calculations were compared with film measurements in a solid water phantom to validate the accuracy of the overall MLC model. MC statistical uncertainty was below 2% for all simulations. Results: We found a good agreement with our measurements on interleaf leakage. Agreement between mean calculated and measured leaf transmissions with fully opened jaws normalized to the center of a 10×10 cm2 field at the same depth was within 0.1%. Discrepancy between MC calculation and measurement for the irregular field was below 2%/2 mm. The gamma analysis of the comparison of MC and EBT2 film measurements in IMRT and VMAT fields showed 99.1%, 99.5% pass rates with 3%/3 mm criteria, respectively. Conclusion: The Agility MLC produced by Elekta could be accurately MC modeled with an adaptation in BEAMnrc. The MC model proved to be applicable for IMRT and VMAT calculations. Japan Society for the Promotion of Science (JSPS) Core‐to‐Core Program