Large energy acceptance beam delivery systems may enable ultra-fast treatment within 10 seconds compared to conventional systems

The availability of proton beam therapy (PBT) continues to grow exponentially worldwide, driven by technological advancements to reduce the facility size and costs, towards more efficient and higher quality treatments. The characteristic physical and biological advantages of protons can provide superior clinical outcomes for patients, as modern techniques enable a highly configurable and conformal dose delivery. Although active scanning methods allow precise beam control, proton beams are highly sensitive to range and motion errors which impact treatment quality. Treatment delivery is largely determined by capabilities of the beam delivery system (BDS), where faster delivery can have many potential benefits including improved dosimetric quality, utility, cost effectiveness, patient throughput and comfort. Despite significant developments in accelerators, delivery methodologies, dose optimisation and more, the energy layer switching time (ELST) is still a persisting limitation in existing beam delivery systems. The ELST can be a major contributor to the irradiation time, leading to increased treatment times, and may require further compensation using optimisation planning approaches, motion mitigation strategies, or active beam modification. This fundamental constraint can be addressed by increasing the narrow energy acceptance range of conventional beamlines to allow a wide range of beam energies to be transported without bottleneck delays due to magnetic field adjustments, therefore minimising ELSTs and enabling ultra-fast delivery (single field within ∼10 s). We review the abundant opportunities offered by this enabling technology: shorter treatment times, reduced motion induced dose degradation, improved effectiveness of motion management techniques, possibilities for volumetric rescanning, bidirectional delivery, novel planning optimisation schemes, and emerging delivery strategies. We overview the design concepts of several large energy acceptance (LEA) proposals, technology requirements, and also discuss the remaining challenges and considerations with realising a LEA system in practice. Although there are multiple avenues requiring further development and study, a large energy acceptance BDS has the potential for significant clinical benefits: ultra-fast delivery offers both immediate improvements to current treatment delivery and enables future possibilities for PBT.