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Monte Carlo Simulation of a 6MV Varian Truebeam Without Flattening Filter Linac

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T Song

T Song1*, L Zhou1, S Jiang2, (1) Southern Medical University, Guangzhou, Guangdong, China (2) University of California, San Deigo, La Jolla, CA

SU-E-T-496 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: To simulate a 6 MV Varian Truebeam linac operating without flattening filter using an existing Phase space file(PhS) with the monte carlo packeage EGSnrc. Methods: An existing PhS file built after simulating the patient-independent part at the first step in traditional monte carlo simulation was employed. With this PhS being a source, the patient-dependent part was simulated and the dose distributions were calculated using EGSnrc. The 6 MV dose calculation were performed for field size of 10x10cm², 20x20cm², 30x30cm² and 40x40cm² in a 54x54x30 cm³ water phantom . The voxel size for the water phantom was 0.6x0.6x0.6cm³. For the 10x10cm² field, surface buildup calculations were performed using 0.6x0.6x0.3cm³ voxels within 1.8cm of the surface. Dose profiles and percent depth dose(PDD) curves were calculated following 1 billion histories for dose profiles and 500 million histories for PDD curves. Simulation results were compared with measured data for validation. Results: Good agreement between simulations and measurements in water was observed. For the PDD curves, 95% of the calculated data points agree within 2% with the experimental measurements for depths between 1.8cm and 30 cm. In the buildup region, the agreement is within 2%, except several points with the deviation of 5% for the 10x10 cm² open field. For the lateral dose profiles, the agreement is less than 3% at the depth of 1.3cm, 5cm, 10cm, 20cm and 30cm inside the 80% field size. Conclusions: It demonstrated that the existing phase space file is accurate and the simulation is reliable to be used for clinical patient dose calculations in future investigations.

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