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Benchmarking of the TOPAS Monte Carlo System Against Phantom Dose Measurements in Proton Therapy

M Testa

M Testa*, J Schuemann, H Paganetti, Massachusetts General Hospital, Boston, MA

WE-C-BRB-7 Wednesday 10:30:00 AM - 12:30:00 PM Room: Ballroom B

To validate the accuracy of Monte Carlo (MC) simulations performed with TOPAS (TOol for PArticle Simulation; to be released to the proton therapy community) against phantom measurements in proton therapy.

We used TOPAS, a new MC simulation tool that provides a user-friendly interface to the Geant4 package, to simulate proton depth-dose curves from the MGH gantry treatment heads in double-scattering mode. Simulations were compared to QA measurements of SOBPs. Field flatness and field symmetry from all clinical options (i.e. combinations of double-scattering system, range modulator wheel and beam current modulation settings) have been simulated and benchmarked against measurements performed with 2D dose detectors. A step-compensator consisting of two sections of different thickness was placed upstream a water phantom in which we measured lateral and longitudinal dose profiles.

For all treatment options TOPAS depth-dose curves are within clinical required accuracy, the range being within +1/-1.5 mm and the modulation within ±3 mm of the requested specifications. SOBP simulations also validate time-dependent geometry and beam current in TOPAS, as the modulator wheel is rotating with the beam current modulation changing at the same time. The field flatness of simulated protons is within ±2% the average dose measured in a plane transverse to the beam axis. Field symmetries show slight discrepancies compared to measured data. In the case of the step-compensator, TOPAS correctly predicts the lateral width of the beam and longitudinal profiles at different transverse positions. In the transverse profiles, the edge scattering effects due to multiple Coulomb scattering were correctly reproduced by TOPAS.

We demonstrated TOPAS accuracy to reproduce measured dose profiles in a variety of homogeneous phantom setups relevant for proton therapy. The accuracy in the benchmarking achieved with TOPAS simulations provides the basis for routine clinical dose calculations.

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