Monte Carlo computation of 3D distributions of stopping power ratios in light ion beam therapy using GATE‐RTion

Purpose This paper presents a novel method for the calculation of three‐dimensional (3D) Bragg–Gray water‐to‐detector stopping power ratio (sw,det) distributions for proton and carbon ion beams. Methods Contrary to previously published fluence‐based calculations of the stopping power ratio, the sw,det calculation method used in this work is based on the specific way GATE/Geant4 scores the energy deposition. It only requires the use of the so‐called DoseActor, as available in GATE, for the calculation of the sw,det at any point of a 3D dose distribution. The simulations are performed using GATE‐RTion v1.0, a dedicated GATE release that was validated for the clinical use in light ion beam therapy. Results The Bragg–Gray water‐to‐air stopping power ratio (sw,air) was calculated for monoenergetic proton and carbon ion beams with the default stopping power data in GATE‐RTion v1.0 and the new ICRU90 recommendation. The sw,air differences between the use of the default and the ICRU90 configuration were 0.6% and 5.4% at the physical range (R80 — 80% dose level in the distal dose fall‐off) for a 70 MeV proton beam and a 120 MeV/u carbon ion beam, respectively. For protons, the sw,det results for lithium fluoride, silicon, gadolinium oxysulfide, and the active layer material of EBT2 (radiochromic film) were compared with the literature and a reasonable agreement was found. For a real patient treatment plan, the 3D distributions of sw,det in proton beams were calculated. Conclusions Our method was validated by comparison with available literature data. Its equivalence with Bragg–Gray cavity theory was demonstrated mathematically. The capability of GATE‐RTion v1.0 for the sw,det calculation at any point of a 3D dose distribution for simple and complex proton and carbon ion plans was presented.