Fabrication, testing, and clinical validation of an epoxy resin-based range shifter for pencil beam scanning proton therapy

BACKGROUND AND OBJECTIVES: Range shifters (RS) are used to treat shallow targets in pencil beam scanning (PBS) proton therapy. This study aims to validate an in-house-fabricated epoxy resin RS through mechanical and dosimetric measurements, evaluate the Monte Carlo (MC) algorithm dose‑calculation accuracy of the treatment planning system (TPS) beam model incorporating this device, and assess its clinical feasibility for PBS proton therapy applications. METHODS: The RS was fabricated using a silicone mould and machined to the required dimensions after curing. It was further evaluated through material characterization, mechanical testing, and dosimetric validation. Material characterization included determining the elemental composition, mass density, and mean excitation energy, which were incorporated into the beam modelling. Mechanical validation comprised dimensional verification, fitment testing, and assessment of material integrity and uniformity. Dosimetric validation was performed by measuring water-equivalent thickness (WET), range parameters, spot size, absolute dose, and fluence, and comparing them with TPS-MC calculations. RESULTS: The cured epoxy resin exhibited normalized elemental composition of H:8.0%, C:65.0%, N:0.1%, and O:26.9%, with mass density 1.15 g/cm³ and mean excitation energy 72.5 eV. The fabricated RS matched the reference design within ±1 mm and demonstrated satisfactory fitment, reproducibility, material integrity and uniformity. The measured WET agreed with TPS estimated values within 0.06 g/cm², while differences in range parameters were <1 mm. Measured spot sizes and the 9-spot matrix showed excellent agreement with TPS estimates across different air-gap settings. Absolute dose measurements agreed with TPS-MC calculations within 3%, and patient-specific fluence measurements showed excellent agreement with TPS calculations under stringent gamma evaluation criteria. CONCLUSION: This study presents the first comprehensive clinical validation and evaluation of cured solid epoxy resin as an RS material for PBS proton therapy applications. The in-house-fabricated, cured epoxy resin-based RS demonstrated excellent agreement with TPS-MC calculations and experimental measurements, establishing its clinical feasibility.