Design and commissioning of divergent proton minibeam collimators and supporting accessories for homogeneous target dose irradiation

OBJECTIVE: Proton minibeam radiation therapy (pMBRT) is a novel spatially fractionated radiation therapy technique that offers improved tumor control and reduced normal tissue toxicity. However, its clinical translation has been hindered in part by limited field sizes, low dose rates, and complex manufacturing. This work describes the design, manufacturing, and commissioning of a novel divergent pMBRT collimator along with accessory components to maximize its versatility and facilitate clinical implementation. APPROACH: The collimator geometry, including slit width, center-to-center (CTC) spacing, thickness, divergence, and field size, was optimized using TOPAS Monte Carlo (MC) to maximize surface peak-to-valley dose ratio (PVDR) while maintaining uniform target coverage. The collimator was constructed using brass plates held by aluminum side holders. Commissioning measurements with a PTW Bragg peak chamber and EBT4 radiochromic film were compared to TOPAS and an in-house fast GPU-based MC simulation. Ancillary equipment consisting of lead foils, range shifters, custom apertures, and secondary pMBRT collimators were characterized in tandem. MAIN RESULTS: An optimized divergent pMBRT collimator with 0.5 mm slit width, 2.0 mm CTC distance, and 40 mm thickness produced a surface PVDR of 4.8 while preserving quasihomogeneous dose in the 6-14 cm depth region. Measured dose profiles demonstrated valley dose agreement within 3% for both TOPAS and GPU-based MC predictions after the simulated collimator geometry was refined to match Bragg-peak chamber average dose measurements. The onset of the homogeneous dose region was shifted toward shallower depths by using thin lead foils, without significantly impacting entrance PVDR. Apertures sharpened penumbra and enhanced peripheral PVDR. A dual collimator approach produced 2D pinhole dose arrays that resulted in valley doses from MC simulations and film measurements agreeing within ~5%. SIGNIFICANCE: We demonstrated a practical low-cost, large-area divergent physical pMBRT collimation system with validated MC modeling. In addition, we introduce complementary accessories that substantially expand system versatility to facilitate clinical implementation.