SU-E-T-496: Monte Carlo Simulation of a 6MV Varian Truebeam Without Flattening Filter Linac

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.

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 10×10cm² , 20×20cm² , 30×30cm² and 40×40cm² in a 54×54×30 cm³ water phantom. The voxel size for the water phantom was 0.6×0.6×0.6cm³ . For the 10×10cm² field, surface buildup calculations were performed using 0.6×0.6×0.3cm³ 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.

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 10×10 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.

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. Project supported by the National Natural Science Foundation of China (30970866).