Treatment planning comparison of proton arc therapy and intensity modulated proton therapy for synchronous bilateral breast or chest wall and regional nodal irradiation

Background and purpose: Bilateral breast or chest wall and regional nodal irradiation involves large treatment volumes adjacent to multiple critical organs. The feasibility and normal tissue sparing of proton arc therapy (PAT) in this setting were evaluated. Materials and methods: PAT and Intensity Modulated Proton Therapy (IMPT) plans were created for twenty breast cancer patients prescribed 50 Gy (RBE) in 25 fractions. Dose-volume parameters and delivery time were compared. Subgroup analyses were performed in patients with thick chest walls (bilateral intact breasts or implants) and thin chest walls (post-mastectomy without reconstruction). Results: Target coverage was comparable between techniques. Compared to IMPT, PAT reduced mean left lung dose by 45.5% (4.8 vs. 8.7 Gy (RBE), p < 0.001) and mean right lung dose by 36.2% (5.1 vs. 8.0 Gy (RBE), p < 0.001). PAT achieved reduction in maximal left anterior descending artery (LAD) dose (3.2 vs. 4.3 Gy (RBE), p = 0.044). In the near-surface region, the mean dose was reduced by 4.1 Gy (RBE) with PAT (40.8 vs. 44.9 Gy (RBE), p < 0.001), and this benefit was consistent in subgroup analyses of patients with thick chest walls (39.9 vs. 44.6 Gy (RBE) p < 0.001) and patients with thin chest walls (43.6 vs. 45.9 Gy (RBE), p = 0.020). PAT also demonstrated significantly shorter average delivery time compared to IMPT (195 vs. 497 s, p < 0.001). Conclusions: PAT reduced dose to the lungs, heart, and near-surface region, while improving delivery efficiency in bilateral breast or chest wall and regional nodal irradiation.