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Life sciences · Journal article
Physics in Medicine and Biology · October 5, 2026
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Abstract Objective. Stereotactic body radiotherapy with a simultaneous integrated boost (SBRT-SIB) for locally advanced pancreatic cancer (LAPC) is constrained by the proximity of serial gastrointestinal organs at risk (OARs). Conventional intensity-modulated proton therapy (IMPT) relies on a limited number of fixed beam angles, which may restrict dose shaping around these critical structures. This study evaluated whether non-coplanar spot-scanning proton arc therapy (SPArc-4π) could improve OAR sparing while preserving robust target coverage. Approach. 10 patients with LAPC were retrospectively selected in this study. For each patient, SBRT-SIB plans were generated using conventional IMPT and SPArc-4π. Plan robustness was assessed across 21 uncertainty scenarios. Target coverage, conformity, OAR doses, estimated delivery time, and Lyman-Kutcher-Burman normal tissue complication probability (NTCP) for four gastrointestinal endpoints were compared using the Wilcoxon signed-rank test. Main results. SPArc-4π maintained target coverage comparable to IMPT, with similar clinical target volume (CTV) and boost-volume coverage, while CTV conformity improved with SPArc-4π (conformity index: 0.50 vs. 0.56; p = 0.02). SPArc-4π significantly reduced maximum dose (Gy(RBE)) to the stomach (30.01 to 24.53), duodenum (35.14 to 33.14), bowel (32.60 to 28.91), and spinal cord (15.38 to 5.58; all p ≤ 0.01). Predicted toxicity decreased across all four gastrointestinal endpoints: gastric bleed risk decreased from 8.19% to 6.15%, grade ≥3 gastrointestinal toxicity from 3.91% to 3.81%, ulceration/perforation from 0.03% to 0.01%, and diarrhea from 24.14% to 22.34% (all p ≤ 0.005). All plans met robustness criteria. The main trade-off was delivery efficiency, with SPArc-4π requiring approximately 2-3 minutes longer delivery time than IMPT. Significance. SPArc-4π provides a promising proton planning strategy for pancreatic SBRT-SIB by preserving robust target coverage while improving gastrointestinal organ sparing and reducing predicted toxicity. These findings support further investigation of SPArc-4π for safer dose escalation in LAPC.