A fully 3D-printed contoured double passive scattering system for ultra-high-dose-rate irradiations

Ultra-High Dose-Rate (UHDR) Proton Therapy is an area of active research due to its potential to target cancer cells while sparing healthy tissues. To deepen the knowledge of underlying biological mechanisms of FLASH effect, pre-clinical experiments are required, necessitating large uniform field achieving dose rates above 40 Gy/s. To achieve this, in the Research & Development (R&D) fixed horizontal proton beam line of HollandPTC, a fully 3D printed contoured passive scattering system has been developed. This system is designed to shape a 250 MeV proton beam into a suitable configuration for pre-clinical radiobiological experiments, achieving the necessary dose rates and uniform field distribution. The beamline configuration was initially modelled using the Monte Carlo-based Tool for Particle Therapy Simulation (TOPAS). Subsequently, the contoured passive scattering system was optimized through simulations to generate a sufficiently uniform field for future radiobiological experiments. To validate simulations, the system was fabricated using advanced 3D printing technology. A tungsten-heavy filament blend, consisting of approximately 75% tungsten by mass, was employed in a fused deposition modelling (FDM) process. Simulations were validated against experimental data. The measured dose distributions demonstrated a lateral field uniformity exceeding 95%, corresponding to a dose homogeneity within ± 3% over a field diameter of 2.8 cm. Dose rates above 40 Gy/s were achieved under experimental conditions, with measured values reaching up to approximately 60 Gy/s at the beam entrance, and 100 Gy/s within the Spread-Out Bragg Peak (SOBP). These results confirm the system's design and performance, thus opening multiple possibilities for UHDR radiobiological experiments.