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Modeling the TrueBeam linac using a CAD to Geant4 geometry implementation: Dose and IAEA‐compliant phase space calculations

2011; Wiley; Volume: 38; Issue: 7 Linguagem: Inglês

10.1118/1.3598439

2473-4209

M. Constantin, Joseph Perl, Tom LoSasso, A. Salop, D.H. Whittum, Anisha Narula, M Svatos, Paul Keall,

Nuclear Physics and Applications

Purpose: To create an accurate 6 MV Monte Carlo simulation phase space for the Varian TrueBeam treatment head geometry imported from cad (computer aided design) without adjusting the input electron phase space parameters. Methods: geant 4 v4.9.2.p01 was employed to simulate the 6 MV beam treatment head geometry of the Varian TrueBeam linac. The electron tracks in the linear accelerator were simulated with Parmela, and the obtained electron phase space was used as an input to the Monte Carlo beam transport and dose calculations. The geometry components are tessellated solids included in geant 4 as gdml (generalized dynamic markup language) files obtained via STEP (standard for the exchange of product) export from Pro/Engineering, followed by STEP import in Fastrad, a STEP– gdml converter. The linac has a compact treatment head and the small space between the shielding collimator and the divergent arc of the upper jaws forbids the implementation of a plane for storing the phase space. Instead, an IAEA (International Atomic Energy Agency) compliant phase space writer was implemented on a cylindrical surface. The simulation was run in parallel on a 1200 node Linux cluster. The 6 MV dose calculations were performed for field sizes varying from 4 × 4 to 40 × 40 cm 2 . The voxel size for the cm 3 water phantom was mm 3 . For the cm 2 field, surface buildup calculations were performed using mm 3 voxels within 20 mm of the surface. Results: For the depth dose curves, 98% of the calculated data points agree within 2% with the experimental measurements for depths between 2 and 40 cm. For depths between 5 and 30 cm, agreement within 1% is obtained for 99% ( ), 95% ( ), 94% ( and ), and 89% ( ) of the data points, respectively. In the buildup region, the agreement is within 2%, except at 1 mm depth where the deviation is 5% for the cm 2 open field. For the lateral dose profiles, within the field size for fields up to cm 2 , the agreement is within 2% for depths up to 10 cm. At 20 cm depth, the in‐field maximum dose difference for the cm 2 open field is within 4%, while the smaller field sizes agree within 2%. Outside the field size, agreement within 1% of the maximum dose difference is obtained for all fields. The calculated output factors varied from for the cm 2 field to for the cm 2 field. Their agreement with the experimental output factors is within 1%. Conclusions: The authors have validated a geant 4 simulated IAEA‐compliant phase space of the TrueBeam linac for the 6 MV beam obtained using a high accuracy geometry implementation from cad . These files are publicly available and can be used for further research.