TOPAS Monte Carlo simulation for double scattering proton therapy and dosimetric evaluation

• Published in: Physica medica Vol. 62; pp. 53 - 62
• Main Authors: Liu, Hongdong, Li, Zuofeng, Slopsema, Roelf, Hong, Liu, Pei, Xi, Xu, Xie George
• Format: Journal Article
• Language: English
• Published: Italy Elsevier Ltd 01.06.2019
• Subjects: Double scattering; Monte Carlo; Proton therapy; Range compensator; TOPAS; TOPAS; Double scattering; Proton therapy; Range compensator; Monte Carlo
• ISSN: 1120-1797; 1724-191X
• DOI: 10.1016/j.ejmp.2019.05.001

•A detailed Monte Carlo based double scattering nozzle model for proton therapy was developed.•Verifications of pristine peaks, spread-out Bragg peaks, lateral profiles in water phantom were performed.•Highly inhomogeneous compensator was investigated to evaluate the dose accuracy by TOPAS and TPS.•Our Monte Carlo model can reproduce more accurate results than TPS calculations. To construct and commission a double scattering (DS) proton beam model in TOPAS Monte Carlo (MC) code. Dose comparisons of MC calculations to the measured and treatment planning system (TPS) calculated dose were performed. The TOPAS nozzle model was based on the manufacturer blueprints. Nozzle set-up and beam current modulations were calculated using room-specific calibration data. This model was implemented to reproduce pristine peaks, spread-out Bragg peaks (SOBP) and lateral profiles. A stair-shaped target plan in water phantom was calculated and compared to measured data to verify range compensator (RC) modeling. TOPAS calculated pristine peaks agreed well with measurements, with accuracies of 0.03 cm for range R90 and 0.05 cm for distal dose fall-off (DDF). The calculated SOBP range, modulation width and DDF differences between MC calculations and measurements were within 0.05 cm, 0.5 cm and 0.03 cm respectively. MC calculated lateral penumbra agreed well with measured data, with difference less than 0.05 cm. For RC calculation, TPS underestimated the additional depth dose tail due to the nuclear halo effect. Lateral doses by TPS were 10% lower than measurement outside the target, while maximum difference of MC calculation was within 2%. At deeper depths inside the target volume, TPS overestimated doses by up to 25% while TOPAS predicted the dose to within 5% of measurements. We have successfully developed and commissioned a MC based DS nozzle model. The performance of dose accuracy by TOPAS was superior to TPS, especially for highly inhomogeneous compensator.