Modeling and validation of proton beam for spot scanning system with GATE software

In this study, we used the Monte Carlo-based software, GATE, to model the spot scanning proton therapy system (SIEMENS IONTRIS spot scanning dedicated nozzle) installed at the Shanghai Proton and Heavy Ion facility. Within the nozzle, apart from entrance and exit windows and the two ion chambers, the beam traverses through the vacuum, allowing for a convergent beam downstream of the nozzle exit. We model the angular, spatial, and energy distributions of the beam phase space at the nozzle exit with single Gaussians, controlled by eight energy-dependent parameters. The parameters were determined from measured profiles and depth dose distributions. Verification of the beam model was done by comparing measured data and GATE acquired relative dose distributions, using plan specific log files from the machine to specify beam spot positions and energy. The MC simulations showed good agreement with measurements for the depth-dose curve and SOBP plans. The absolute comparison of the absorbed dose difference between the MC and the measurement was ±1%. This work describes a method for adapting a MC simulation model for a spot scanning proton delivery system. The excellent agreement between the measurements and simulations shows that the MC modeling in this work is a precise and reputable method.