Proton Therapy Reduces Cardiopulmonary Dose in Synchronous Bilateral Breast Cancer, Especially with Direct Chest Wall Irradiation.

Purpose/Objective(s): To evaluate the dosimetric advantages of intensity-modulated proton therapy (IMPT) compared to volumetric modulated arc therapy (VMAT) in patients with synchronous bilateral breast cancer (sBBC), with a focus on cardiopulmonary sparing, particularly in patients receiving direct chest wall irradiation. Methods and Materials: A retrospective dosimetric analysis was conducted on 11 patients with sBBC treated with both IMPT and VMAT plans between 2020 and 2021 at a tertiary cancer center. Target volumes included bilateral breasts/chest wall and nodal regions. Mean dose V5, and V10, values were compared for the heart, lungs, and left anterior descending artery (LAD) using Wilcoxon signed-rank tests. V20 was compared for the heart and lungs. A subanalysis was performed for patients who received direct chest wall radiation. Results: IMPT significantly reduced mean heart dose (−522 cGy, p = 0.0034), heart V5 (−67.77%, p = 0.0076), and heart V10 (−0.91%, p = 0.0164). Lung exposure was also lower with IMPT: mean dose (−988 cGy, p = 0.0034), V5 (−52.38%, p = 0.0044), V10 (−30.21%, p = 0.0044), and V20 (−19.68%, p = 0.0034). When looking at the left anterior descending artery (LAD), IMPT significantly reduced mean LAD dose (−509 cGy, p = 0.0034), V5 (−88.12%, p = 0.0069), and V10 (−11.47%, p = 0.0093). In the chest wall subcohort, IMPT showed greater sparing of heart (−792 cGy), lungs (−820 cGy), and LAD (−1117 cGy), all with statistically significant Z-scores. Conclusion: IMPT significantly reduces radiation dose to the heart, lungs, and LAD in patients with sBBC compared to VMAT, particularly in those receiving direct chest wall irradiation. These findings support expanding access to proton therapy and suggest that current insurance approval criteria should include chest wall radiation as an indication.