Systematic Optimization of Slit Collimator Design for MV Photon Minibeam Radiation Therapy
Abstract
Purpose
Minibeam radiation therapy (MBRT) is a spatially fractionated technique that produces millimeter-scale dose heterogeneity with promising normal tissue–sparing effects. Implemented using metallic collimators mounted on the accessory tray, MBRT can be delivered on conventional clinical linear accelerators. However, MBRT dosimetric performance is highly sensitive to collimator design. This study uses Monte Carlo simulations to evaluate MBRT collimator designs and quantify the effects of slit geometry, collimator material and thickness, and air gap.
Methods
Monte Carlo simulations were performed using GATE to model a 6 MV photon beam from a clinical linear accelerator irradiating a water phantom. The collimator was positioned at a source-to-collimator center distance of 425 mm, with the slit geometry designed to account for beam divergence. Three categories of slit collimator parameters were investigated: slit geometry (slit widths 0.5–2.0 mm; slit-to-blade ratios R = 0.1–1.0), collimator material and thickness (tungsten, tungsten alloy, lead, brass, stainless steel-316, and aluminum; 60–200 mm), and air gap (collimator-to-surface distances of 50–500 mm). Peak-to-valley dose ratio (PVDR), full width at half maximum (FWHM), peak dose, and valley dose were evaluated at clinically relevant depths.
Results
PVDR was strongly dependent on slit geometry. Within this multi-parameter design space, the Pareto front indicates that a maximum PVDR of 18.7 was achieved with a slit width of 2.0 mm and R=0.2, whereas more alternative configurations (1.0 mm, R=0.8) yielded a PVDR of 7.4. High-density materials (tungsten and lead) achieved near-optimal PVDR values (~15–17) at 60 mm thickness, while medium-density materials (brass and stainless steel) required approximately 100 mm; aluminum was inadequate even at 200 mm. Air gap effects were non-monotonic, with PVDR peaking at 17.5 at 200 mm.
Conclusion
Monte Carlo–based optimization defines a clinically feasible MBRT collimator design envelope that balances dosimetric performance with manufacturability, supporting practical MBRT delivery on conventional linear accelerators.