VIT-MBRT, a GPU accelerated Monte Carlo tool for investigations on preclinical minibeam radiation therapy

Purpose: Minibeam Radiation Therapy (MBRT) is a form of spatially fractionated radiation therapy (SFRT) with electrons, protons, ions or kilovoltage photon beams, aiming to improve the therapeutic window compared to broad-beam irradiation. MBRT with X-rays uses an orthovoltage source and an array of narrow beamlets produced by collimator slits with a typical width of 0.2 mm to 0.7 mm and a center-to-center spacing of 1 to 4 mm. We developed an updated version of gCTD, a fast Monte Carlo (MC) code, initially designed for cone-beam CT imaging, to implement a treatment planning system for photon MBRT in mice within a virtual imaging and dosimetry tool (VIT-MBRT). Methods: We compared gCTD, running on a single GPU platform (CUDA environment) with TOPAS, a well validated CPU-based simulation code. We simulated irradiation of a water phantom using a multi-slit tungsten collimator with different aperture width and thickness, producing absorbed dose volume data. An example treatment plan was implemented with the gCTD code, using a voxelized mouse phantom derived from a CT scan and evaluating organ dose-volume histograms. Results: Comparison with TOPAS simulations showed a good agreement in terms of dose values and peak-to-valley dose ratio (maximum absolute discrepancy of 13% at the phantom entrance surface), with a few percent differences in depth. Simulations with gCTD achieved a 300-fold reduction in computational time with respect to corresponding TOPAS simulations. Conclusions: We realized and validated a fast GPU-based MC simulation code for minibeam radiotherapy, as the basis of the MC platform VIT-MBRT for kilovoltage MBRT preclinical treatments.