A Novel Degrader System for Ultra-Fast Beam Delivery In Proton Therapy
Abstract
Purpose
Slow energy switching remains a limitation in proton therapy. This study aims to design and optimize an energy degrader system to accelerate layer-to-layer dose delivery in compact proton therapy systems, thereby improving treatment efficiency, outcomes, and access to proton therapy.
Methods
The proposed ultra-fast energy degrader enables ultra-fast beam delivery and can be paired with large momentum acceptance beamlines or fast-ramping quadrupoles. We focused on a gantry-less system incorporating fixed-field alternating-gradient (FFA) optics. The FFA beamline enables the transport of beam energies covering the full therapeutic range without adjusting magnet settings. Consequently, the degrader solely determines the energy-switching speed, while the system footprint is reduced. To minimize emittance growth and maximize transmission, Monte Carlo simulations were performed to optimize the degrader and phase-space collimators. Different materials and wedge-based geometries were considered for the degrader, whereas collimator optimization focused on configuration, shape, length, and spacing. The design was iteratively refined to achieve a compact and efficient system.
Results
The design with two boron carbide wedges moving in opposite directions was optimal. A wedge angle of 20.6° allows maximum exit energies of 216.6 MeV and 237.3 MeV for 230 MeV and 250 MeV incident proton beams, respectively. Each wedge weighs 1 kg and requires displacements up to 3 mm to cover all typical energy-layer configurations, allowing movement within 10 ms using commercially available motors. Combined with a two-collimator setup spaced 25 cm apart, this design maintains emittances below 20 mm·mrad and a minimum transmission of 2% at 70 MeV.
Conclusion
The proposed ultra-fast degrader can deliver a single field in seconds, considerably reducing treatment times. It benefits moving targets and advanced techniques like FLASH and proton arc therapies, enhances patient comfort, and increases facility throughput. The design is system-agnostic and can be integrated into existing cyclotron- and synchrotron-based proton therapy systems.