Particle Arc Therapy in Cancer Radiotherapy: A Scoping Review of Feasibility and Dosimetric Evidence.

Background/Objectives: Particle arc therapy (PArc) is an emerging radiotherapy technique with the potential to improve dose conformity and organ-at-risk (OAR) sparing, while also enhancing biological effectiveness through favourable dose-averaged linear energy transfer (LETd) distributions. This scoping review aimed to evaluate the current evidence on PArc, including dosimetric, biological, and delivery-related outcomes, and to identify key limitations and future research directions. Methods: Following the PRISMA-ScR 2018 guidelines, a systematic search was conducted across MEDLINE, Scopus, and Google Scholar in December 2025, identifying 74 relevant studies. Eligible studies included peer-reviewed articles reporting treatment planning, dosimetric, biological, or delivery outcomes, with comparisons to conventional particle therapy techniques. Data were extracted and synthesised qualitatively due to heterogeneity in study designs, reported metrics, and outcome definitions. Results: Relative to conventional particle therapy techniques, PArc generally demonstrated maintained target coverage, enhanced conformity (median relative improvement in conformity index: 6.3%), and more variable dose homogeneity. Improved OAR sparing and reduced integral dose (median relative reduction in integral dose: 10.5%) were commonly observed, although these benefits were sometimes accompanied by an increased low-dose bath, reflecting a trade-off between low-dose and intermediate-high-dose regions. Reductions in intermediate-to-high dose volumes were often associated with improved modelled biological outcomes, including reduced predicted normal tissue complication probabilities and secondary cancer risk. Enhanced LETd distributions were reported, with redistribution of high LETd from surrounding healthy tissue to the tumour volume, particularly in studies of carbon-ion arc therapy and radioresistant tumours. Robustness findings were mixed, with trade-offs among conformity, homogeneity, delivery efficiency, and LETd enhancement, and strong dependence on tumour site, delivery strategy, and evaluation method. Delivery efficiency was also variable, depending on machine capabilities and optimisation strategies. Conclusions: PArc shows significant potential to improve radiotherapy outcomes; however, the current evidence is largely limited to in silico studies. Further research is required to establish standardised methodologies and evaluation frameworks, integrate biological optimisation, and validate findings through prospective clinical studies before routine clinical implementation can be achieved.