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

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 (