Toward Ultra-fast Treatments: Large Energy Acceptance Beam Delivery Systems and Opportunities for Proton Beam Therapy.

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 BDS. The ELST can contribute significantly to beam delivery time (BDT) and extend treatment times, requiring compensation by 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 minimise the ELST, enabling ultra-fast delivery. A large energy acceptance (LEA) BDS has the potential to revolutionise PBT through immediate improvements to current treatment delivery and emerging delivery modalities: the complete exploitation of PBT - and unlocking its full potential - can only be made possible with advances in beam delivery technologies. We review the abundant opportunities offered by an ultra-fast BDS: shorter treatment times, reduced motion induced dose degradation, improved effectiveness of motion management techniques, possibilities for volumetric rescanning, bidirectional delivery, further planning optimisation, and novel delivery strategies. We overview the design concepts of several LEA proposals, technology requirements, and also discuss the remaining challenges and considerations with realising a LEA BDS in practice. There are multiple avenues requiring further development and study, however the clinical potential and benefits of this enabling technology are clear: ultra-fast delivery offers both immediate and future improvements to PBT treatments.