Optimizing minibeam collimator design for enhancing normal tissue sparing in ocular tumour proton therapy

Uveal melanoma, the most common primary intraocular tumour in adults, presents significant therapeutic challenges due to its aggressive nature and the potential for severe treatment-related complications, including vision loss. Achieving effective tumour control while minimizing damage to surrounding healthy tissues remains a critical goal in radiotherapy. Proton minibeam radiotherapy (pMBRT), an advanced form of spatially fractionated radiotherapy (SFRT), has emerged as a promising approach to address these challenges. pMBRT employs a mechanical collimator to spatially fractionate a broad proton beam into multiple narrow beamlets, creating a dose distribution with high-dose peaks and low-dose valleys in shallow regions. As the beamlets travel deeper into tissue, multiple Coulomb scattering facilitates their convergence, resulting in a uniform dose at the tumour target. This study systematically optimized the collimator design by evaluating various geometries and materials, specifically brass and polylactic acid (PLA). Simulations of dose distributions were performed using the Tool for Particle Simulation (TOPAS) and validated through experimental measurements with Gafchromic films. Results indicated that brass collimators, with their high atomic number, produced sharper dose profiles and higher peak-to-valley dose ratios (PVDR), demonstrating superior spatial dose modulation. Conversely, PLA collimators yielded smoother dose profiles and lower secondary dose contributions, showcasing their potential for reducing collateral tissue damage. The optimized collimator design, featuring a 0.8 mm slit width and a 1 mm spacing, achieved an ideal balance between maximizing PVDR and ensuring uniform beam recombination at the target depth. These findings underscore the potential of tailored collimator designs to enhance the therapeutic precision of pMBRT, offering improved tumour control with minimized impact on healthy tissues. This study provides a foundation for further advancements in collimator technology and its clinical applications in treating uveal melanoma and other challenging tumour sites.