Spatially fractionated radiotherapy with X-ray minibeams (x-MBRT) aims to increase normal-tissue tolerance by delivering alternating high- and low-dose regions. We provide a Monte Carlo-based framework to design and optimize multi-slit collimators, quantifying how geometry and material govern peak–valley modulation. A validated digital twin of the SmART X-RAD225Cx irradiator was implemented in TOPAS/Geant4. Various x-MBRT collimators were simulated with parallel or divergent slits. The parameter space covered a slit width w (0.1–0.9 mm), center-to-center spacing CTC (1–3 mm), thickness T (1–5 mm), and acceptance angle θ. Dose was scored in a 2 × 2 × 2 cm3 water phantom at a 1 cm depth. For fixed w/CTC, peak-valley dose ratio PVDR increases with larger CTC via an increase in peak dose, with the valley dose nearly constant. Peak transmission saturated at θ ≈ 3°, indicating minimal benefit from larger acceptance. Divergent slits yielded flatter lateral profiles but higher valley doses than parallel slits, reducing PVDR around the central axis. This Monte Carlo study provides insights for optimizing collimator geometries in x-MBRT using small-animal irradiators, informing the design of more effective collimation systems to enhance treatment precision and normal-tissue sparing.
Crimaldi et al. (Sat,) studied this question.