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

TL;DR

This article argues that the energy layer switching time (ELST) bottleneck in current proton pencil beam scanning limits treatment speed and throughput. By expanding the beamline momentum acceptance to create a large energy acceptance (LEA) BDS, energy changes can occur with minimal delays, enabling ultra-fast delivery, improved motion management, and new delivery modalities such as volumetric rescanning and bidirectional scanning. The paper surveys LEA concepts, design options for large-energy-acceptance arcs, accelerator and magnet technologies, and the challenges of clinical implementation, including beam quality and QA requirements. While no LEA-system has been clinically realized yet, the authors contend that LEA BDS could substantially reduce treatment times, lower costs, and unlock advanced PBT techniques, making widespread adoption more feasible in the long term. The realization of LEA BDS will require coordinated advances across accelerator physics, magnet technology, degrader design, and TPS integration, along with thorough clinical validation of safety and dose fidelity.

Abstract

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.

Figures (12)

• Figure 1: Worldwide PBT and carbon ion beam therapy (CIBT) facilities: operational, anticipated (under construction) and proposed (planned), total number of CPT facilities listed above bars. PTCOG data updated Dec 2025 PTCOGfacilities. Single room PBT facilities are those reported with one beam or gantry -- including dedicated ocular and upright centers.
• Figure 2: Compact, single-room PBT systems with gantries (left) and upright systems (middle, right). Hitachi Rossi2022, P-Cure and MedAustron synchrotron solutions (top). ProNova and Mevion upright solutions with the Leo Cancer Care Marie chair (bottom, right). Images reproduced with permission, P-cure, ProNova Solutions and Mevion Medical Systems, pending permission from MedAustron.
• Figure 3: Schematic showing typical PBS delivery. 3D coverage of the treatment volume is achieved by scanning the beam across a layer (in the X--Y plane) in a predetermined pattern before the energy is lowered reaching a shorter depth (in Z), then scanned across and repeated again for each consecutive layer. Adapted from Yap2021.
• Figure 4: Beams with a narrow (0.05%, top left) and wide (3.5%, bottom left) incident energy spread (ES) and their resulting spread out BPs (right). A shallower surface dose and sharper distal fall-off can be achieved with a smaller beam energy spread. Reproduced with permission Hsi2009.
• Figure 5: Magnet ramping sequence (shown for rescanning), conventionally from high to low energies, adapted from Actis2018. Schematic of a hysteresis loop showing the field current correlation for an electromagnet. Stable delivery follows the cycle from maximum to minimum energy when delivering layers (blue circles). Delivering on the ascending side of the loop (orange circles) is performed for energy meandering, a novel delivery strategy Actis2023.