LAUSR

BACKGROUND Monte Carlo (MC) simulations are considered the gold-standard for accuracy in radiotherapy dose calculation; however, general purpose MC engines are computationally demanding and require long runtimes. For this reason, several groups have recently developed fast MC systems dedicated mainly to photon and proton external beam therapy, affording both speed and accuracy. PURPOSE To support research and clinical activities at the Heidelberg Ion-beam Therapy Center (HIT) with actively scanned helium ion beams, this work presents MonteRay, the first fast MC dose calculation engine for helium ion therapy. METHODS MonteRay is a CPU MC dose calculation engine written in C++, capable of simulating therapeutic proton and helium ion beams. In this work, development steps taken to include helium ion beams in MonteRay are presented. A detailed description of the newly implemented physics models for helium ions, for example, for multiple coulomb scattering and inelastic nuclear interactions, is provided. MonteRay dose computations of helium ion beams are evaluated using a comprehensive validation dataset, including measurements of spread-out Bragg peaks (SOBPs) with varying penetration depths/field sizes, measurements with an anthropomorphic phantom and FLUKA simulations of a patient plan. Improvement in computational speed is demonstrated in comparison against reference FLUKA simulations. RESULTS Dosimetric comparisons between MonteRay and measurements demonstrated good agreement. Comparing SOBPs at 5, 12.5, and 20 cm depth, mean absolute percent dose differences were 0.7%, 0.7%, and 1.4%, respectively. Comparison against measurements behind an anthropomorphic head phantom revealed mean absolute dose differences of about 1.2% (FLUKA: 1.5%) with per voxel errors ranging from -4.5% to 4.1% (FLUKA: -6% to 3%). Computed global 3%/3 mm 3D-gamma passing rates of ∼99% were achieved, exceeding those previously reported for an analytical dose engine. Comparisons against FLUKA simulations for a patient plan revealed local 2%/2 mm 3D-gamma passing rates of 98%. Compared to FLUKA in voxelized geometries, MonteRay saw run-time reductions ranging from 20× to 60×, depending on the beam's energy. CONCLUSIONS MonteRay, the first fast MC engine dedicated to helium ion therapy, has been successfully developed with a focus on both speed and accuracy. Validations against dosimetric measurements in homogeneous and heterogeneous scenarios and FLUKA MC calculations have proven the validity of the physical models implemented. Timing comparisons have shown significant speedups between 20 and 60 when compared to FLUKA, making MonteRay viable for clinical routine. MonteRay will support research and clinical practice at HIT, for example, TPS development, validation and treatment design for upcoming clinical trials for raster-scanned helium ion therapy.