TOPAS (TOol for PArticle Simulation) is a particle simulation code recently developed with the specific aim of making Monte Carlo simulations user-friendly for research and clinical physicists in the particle therapy community. The authors present a thorough and extensive experimental validation of Monte Carlo simulations performed with TOPAS in a variety of setups relevant for proton therapy applications. The set of validation measurements performed in this work represents an overall end-to-end testing strategy recommended for all clinical centers planning to rely on TOPAS for quality assurance or patient dose calculation and, more generally, for all the institutions using passive-scattering proton therapy systems.
The authors systematically compared TOPAS simulations with measurements that are performed routinely within the quality assurance (QA) program in our institution as well as experiments specifically designed for this validation study. First, the authors compared TOPAS simulations with measurements of depth-dose curves for spread-out Bragg peak (SOBP) fields. Second, absolute dosimetry simulations were benchmarked against measured machine output factors (OFs). Third, the authors simulated and measured 2D dose profiles and analyzed the differences in terms of field flatness and symmetry and usable field size. Fourth, the authors designed a simple experiment using a half-beam shifter to assess the effects of multiple Coulomb scattering, beam divergence, and inverse square attenuation on lateral and longitudinal dose profiles measured and simulated in a water phantom. Fifth, TOPAS’ capabilities to simulate time dependent beam delivery was benchmarked against dose rate functions (i.e., dose per unit time vs time) measured at different depths inside an SOBP field. Sixth, simulations of the charge deposited by protons fully stopping in two different types of multilayer Faraday cups (MLFCs) were compared with measurements to benchmark the nuclear interaction models used in the simulations.
SOBPs’ range and modulation width were reproduced, on average, with an accuracy of +1, −2 and ±3 mm, respectively. OF simulations reproduced measured data within ±3%. Simulated 2D dose-profiles show field flatness and average field radius within ±3% of measured profiles. The field symmetry resulted, on average in ±3% agreement with commissioned profiles. TOPAS accuracy in reproducing measured dose profiles downstream the half beam shifter is better than 2%. Dose rate function simulation reproduced the measurements within ∼2% showing that the four-dimensional modeling of the passively modulation system was implement correctly and millimeter accuracy can be achieved in reproducing measured data. For MLFCs simulations, 2% agreement was found between TOPAS and both sets of experimental measurements. The overall results show that TOPAS simulations are within the clinical accepted tolerances for all QA measurements performed at our institution.
Our Monte Carlo simulations reproduced accurately the experimental data acquired through all the measurements performed in this study. Thus, TOPAS can reliably be applied to quality assurance for proton therapy and also as an input for commissioning of commercial treatment planning systems. This work also provides the basis for routine clinical dose calculations in patients for all passive scattering proton therapy centers using TOPAS.
This work was supported by NCI Grant Nos. R01-CA140735 and P01-CA21239.
The authors wish to thank Thomas Botticello and Tom Ruggieri from MGH for technical support during measurements and for providing QA data.
II.A. Description of the Gantry nozzles in double scattering mode and general TOPAS simulation settings
II.B. Comparison between TOPAS simulations and quality assurance measurements
II.B.1. Depth-dose curves measurements and simulations of pristine peaks and SOBP fields
II.B.2. Output factors measurements and simulations for absolute dosimetry
II.B.3. Two-dimensional dose profile measurements and simulations
II.C. Comparison between TOPAS simulations and dedicated experimental validation measurements
II.C.1. Effects of the half beam shifter on lateral and longitudinal dose profiles
II.C.2. Measurements and simulations of dose rate functions
II.D. Comparison between TOPAS simulations and multilayer Faraday cup (MLFC) measurements
III. RESULTS AND DISCUSSION
III.A. Comparison between TOPAS simulations and quality assurance measurements
III.A.1. Depth-dose curves measurements and simulations of pristine peaks and SOBP fields
III.A.2. Output factor measurements and simulations for absolute dosimetry
III.A.3. Two-dimensional dose profile measurements and simulations
III.B. Comparison between TOPAS simulations and dedicated benchmarking measurements
III.B.1. Effects of a half beam shifter on lateral and longitudinal dose profiles
III.B.2. Measurements and simulations of dose rate functions (DRFs)
III.C. Comparison between TOPAS simulations and multilayer Faraday cup measurements
IV. SUMMARY AND CONCLUSIONS
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