First Clinical Implementation of Step-and-Shoot Proton Arc Therapy for Head and Neck Cancer Treatment

Dynamic Spot-scanning Proton Arc (SPArc -Dynamic ) therapy has gained attention for enhancing dosimetric plan quality. However, its full clinical implementation remains under development. As an interim milestone, we developed step-and-shoot arc therapy (SPArc -step&shoot ) for head-neck cancer treatment. An in-house spot and energy-layer sparsity optimization algorithm was integrated into a clinical treatment planning system. The algorithm prioritized higher MU-weighted energy layers and spots to ensure delivery efficiency and superior plan quality while meeting machine requirements (≥0.02MU/spot). A Dynamic SPArc simulator calculated delivery times, and a machine-learning-based synthetic CT(synCT) platform monitored dose robustness. In June 2024, a head-neck cancer patient with parotid gland malignancy was treated using SPArc -step&shoot (6600 cGy[relative biological effectiveness] in 33 fx) with 9 static fields at 20-degree intervals. Comparative plans (SFO-IMPT, SPArc -Dynamic ) were evaluated for dose metrics, delivery times, and adaptive planning. SPArc -step&shoot and SPArc -Dynamic showed similar target coverage and organ-at-risks sparing, and the plan quality is superior to the 3-field SFO-IMPT in the brainstem, oral cavity, and spinal cord sparing. The simulated continuous arc delivery time is 15.9, 6.32, and 4.31 minutes for SPArc -step&shoot, SFO-IMPT, and SPArc -Dynamic , respectively. The actual recorded average treatment delivery time for SPArc -step&shoot in 33 fx is 16.7 ± 1.56 minutes. QA-CT and synCT showed a similar target coverage degradation and perturbation, and a replan was initiated. The SPArc -step&shoot therapy was successfully implemented in the clinical settings, and first patient was successfully treated between June and August 2024. The synCT platform serves a critical role in the daily monitoring process as SPArc -Dynamic might be more sensitive to the patient geometry changes in HNC treatment.