Optimizing Ridge Filter Design: Delivering Effective Bragg Peak Flash Proton Beam Therapy
Abstract
Purpose
To assess how ridge filter (RF) and range shifter (RS) composition, along with pin spacing, shape, and lattice arrangement, influence in-field dose rates, dose uniformity, and field penumbra widths during ultra-high dose rate proton FLASH treatments. This study also evaluates the robustness of RFs to pencil beam variations and validates simulations using a 3D-printed RF for experimental measurements.
Methods
RFs were designed to account for variable fluence incident upon each pin stack and the contributions from neighboring pencil beams. This method was validated with Monte Carlo simulations, and intended dose distributions were delivered in-silico to water phantoms using various RF–RS systems and designs fabricated with different materials. RFs were 3D-printed from polypropylene-like resin, placed in a 245 MeV Varian ProBeam® beamline, and integrated depth dose (IDD) was measured.
Results
Pin shape (square, hexagonal, circular) and lattice pattern (square, hexagonal) had minimal effect on dose rates and distributions. Polypropylene RF–RS systems produced dose rates notably higher than lead, beryllium, aluminum, and water. RF-induced beam broadening enabled sparsely (>7 mm) spaced pencil beams, and therefore ridge filter pin stacks, to form uniform dose distributions in a water phantom, with such spacings giving the potential to improve dose rates; however, pencil beam-to-RF pin misalignments of just 1 mm can lead to large distortions in longitudinal dose-distributions when beams are sparsely spaced. Robustness was improved by reducing pencil beam spacings, using 2×2 pin stacks per beam, or placing the RF downstream of RS. Irradiation of a 3D-printed RF–RS system accurately generated the expected dose distribution.
Conclusion
Robust RF design is an important consideration due to pencil beam width variations and misalignments. Polypropylene RFs placed downstream of the RS offer high dose rates and robust performance with minimal penumbra broadening. Widely spaced beams can preserve dose uniformity and may boost dose rates.