Clinical viability of an active spot scanning beam delivery system with a newly developed carbon-ion treatment planning system

Purpose Although active spot scanning irradiation technique is theoretically superior to passive-scattered broad beam irradiation with respect to normal tissue sparing, corroborations of the clinical benefit of carbon-ion spot scanning have remained scarce. This study aims to investigate the feasibility and clinical implementation of an active spot scanning beam calculation algorithm in a homemade carbon-ion treatment planning system (TPS) by comparing it with a conventional passive uniform scanning technique. Materials and methods Carbon-ion plans were initially formulated using spot/uniform scanning methods in 22 participants enrolled in a prospective observational clinical trial. Subsequently, two additional plans were designed, resulting in three carbon-ion plans for each participant: uniform and spot scanning with mini-ridge filters (MRFs) of 2mm and 4mm, respectively. Results The findings revealed no significant differences in dose homogeneity; however, significant differences in dose conformity were found between the active and passive scanning plans. For dose drop-off outside the target volume, the average gradient index (GI) values were 1.94 (95% confidence interval (CI): 1.79%-2.09%), 1.87 (95% CI: 1.73%-2.01%), and 3.20 (95% CI: 2.80%-3.61%) for the MRF4, MRF2, and uniform scanning plans, respectively. The pre-treatment tumor volume was 124.7 cm3 (range, 54.2-234 cm3), and the average shrinkage observed was 38.4% (95% CI: 17.6%-59.4%). Seven participants experienced grade 1 acute toxicity, and four experienced grade 2 acute toxicity. However, none of the patients developed grade 3 acute toxicity. Conclusions Increasing evidence suggests that potential clinical advantages of spot scanning delivery underlie its technical characteristics. As one among the few institutions currently utilizing carbon-ion radiotherapy, the investigation also provides promising safety and efficacy outcomes from the initial groups of treated participants, thereby contributing to the established clinical evidence supporting the effectiveness and superiority of carbon-ion therapy.