Impact of center‐to‐center distance and slit width on robust target coverage and PVDR for proton minibeam radiation therapy using a commercial TPS

Abstract Background Proton minibeam radiation therapy (pMBRT) is a novel technique that may reduce normal tissue toxicity through spatial fractionation of proton beams. Purpose This study investigates the influence of multislit collimator (MSC) slit width and center‐to‐center (CTC) distance on robust target coverage and peak‐to‐valley dose ratio (PVDR) in single‐beam pMBRT. Methods Simulations were performed in RayStation 2024B research version using a Monte Carlo (MC) dose engine including MSCs. A homogeneous cubic water phantom with a spherical clinical target volume (CTV) of diameter 6.0 cm was modelled. Nineteen pMBRT plans were generated with slit widths of 0.04–0.20 cm and CTC distances of 0.2–0.6 cm, plus an open beam reference. Four single‐beam geometries were simulated, representing different entrance‐to‐target depths (6–24 cm) and angles. Robust composite minmax optimization (CMRO) with 28 scenarios (5 mm setup, 3% range uncertainty) was applied. Robust target coverage and PVDR at beam entrance and CTV center were analyzed. Tumor control probability (TCP) was calculated for illustrative purposes. Results A trade‐off was observed between entrance PVDR and robust target coverage. High PVDR values at entrance reduced coverage, while sufficient coverage reduced PVDR. CTV depth and beam geometry were key factors: larger depths and oblique angles reduced entrance PVDR ( < 2.7) and coverage. Intermediate depths (6–15 cm) provided the most favorable balance. TCP was comparable between pMBRT and open beams when PVDR in the target was < 1.2, but decreased with heterogeneous target doses. Conclusions This study demonstrates that robust single‐beam pMBRT planning in RayStation is feasible but can be limited by a trade‐off between PVDR and robust target coverage, and heavily dependent on beam depth and geometry. These findings provide insights into the effects of beam geometry and collimator parameters on PVDR and robustness, guiding future investigations on robust pMBRT planning.