Robustness of 4D-optimized intensity-modulated proton radiation therapy and proton arc radiation therapy for stereotactic arrhythmia radio-ablation against cardiorespiratory motion

Background Stereotactic arrhythmia radioablation (STAR) is an emerging noninvasive treatment for refractory ventricular tachycardia. Proton therapy offers dosimetric advantages, but pencil-beam scanning is motion-sensitive. This study used an extended cardiac-torso (XCAT)-based 4D dynamic dose (4DDD) framework to compare motion-induced changes between 4D-robustly optimized intensity-modulated proton therapy (IMPT) and proton arc therapy (PAT). Methods Twenty-four XCAT-based virtual patients were generated by varying sex, ejection fraction, respiratory motion, and left ventricular (LV) motion amplitude. IMPT and PAT plans used 4D robust optimization (4DRO). Nominal dose and 4DDD evaluations were compared for clinical target volume (CTV) coverage and organ-at-risk (OAR) doses. LV segment-specific analyses were performed. Volume rescanning scenarios of 1r, 2r, and 4r were simulated. Results Both techniques achieved adequate CTV coverage under nominal dose evaluation. Under 4DDD evaluation, IMPT showed greater degradation than PAT, with D 95 decreasing from 25.6 ± 0.3 to 24.7 ± 0.8 Gy (RBE) and V 25 from 99.5 ± 0.9% to 86.4 ± 13.9%. PAT showed smaller changes, with D 95 decreasing from 25.5 ± 0.2 to 25.4 ± 0.4 Gy (RBE) and V 25 from 99.8 ± 0.4% to 98.2 ± 3.2%. Segments 4, 5, and 6 showed greater degradation; Segments 15 and 16 were more stable. Rescanning improved CTV coverage and homogeneity. Conclusions Within the investigated XCAT scenarios, PAT showed smaller motion-related reductions in CTV coverage than IMPT after 4DRO. For PAT, two additional rescans (2r; three total deliveries) maintained stable CTV metrics under the tested starting-phase arrangement, while IMPT showed greater benefit from rescanning.