2922 - Landscape of Cardiac Substructure Dose Reduction with IMPT vs. IMRT and Estimated Cardiac Toxicity Difference Using Two NTCP Models in Breast Cancer Patients Receiving Postoperative Radiotherapy
Presenter(s)
L. Cao1,2, H. Zhao1,2, S. Zhang1,2, K. Liao1,2, M. Chen1,2, G. Cai1,2, D. Ou1,2, J. Yang1,2, X. Y. Wu1,2, H. Li1,2, F. F. Xu1,2, W. Qi1,2, Y. B. Zhang1,2, and J. Y. Chen1,2; 1Department of Radiation Oncology, Ruijin Hospital, Shanghai Jiaotong University School of Medicine, Shanghai, China, 2Shanghai Key Laboratory of Proton Therapy, Shanghai, China
Purpose/Objective(s): Intensity-modulated proton therapy (IMPT) has demonstrated significant reductions in whole heart dose for breast cancer patients undergoing postoperative radiotherapy (RT). This study evaluated the magnitude of dose reduction based on cardiac substructure between IMPT and intensity-modulated radiotherapy (IMRT). The associated risk of cardiac toxicity was also assessed using two different NTCP models.
Materials/Methods: We reviewed 199 breast cancer patients from a randomized trial (NCT02942615) who received IMRT, grouping them by tumor laterality and target volume. The top five patients with the highest mean heart dose (MHD) from each group were included. IMPT plans were generated for all 30 patients with a prescribed dose of 4005 cGy(RBE) in 15 fractions. Cardiac substructures were contoured, and dosimetric parameters were collected. The normal tissue complication probability (NTCP) models by Darby et al. and van den Bogaard et al. were used to compare the cumulative risk of acute coronary events (ACE) between IMPT and IMRT.
Results: IMPT significantly reduces the dose to all cardiac substructures with a magnitude from 63.34% to 100%, with greater absolute reductions in left-sided and relative reductions in right-sided patients. For left-sided internal mammary node irradiation (IMNI), IMPT achieved an 82.25% reduction for LAD (P = 0.009), 79.45% for RV (P < 0.001), and over 90% for other substructures. Right-sided patients had near-zero mean doses in most substructures. In left-sided patients, the substructure with the highest remaining dose was the LAD, especially in IMNI cases, followed by the left ventricle (LV) and RV. For right-sided patients, only limited maximum dose to the right atrium (RA), right coronary artery (RCA), and right ventricle (RV) found in IMNI. The Darby model shows that IMPT reduces individual ACE risk by 1.58% to 5.16% for left-sided RNI with IMNI (P = 0.001) and 0.59% to 1.05% for right-sided RNI with IMNI (P = 0.063), based on cardiovascular risk factors. The Bogaard model shows that IMPT reduces 9-year individual ACE risk by 0.51% to 0.88% based on LV_V5 (P = 0.002) and by 0.19% to 2.75% based on MHD in left-sided RNI with IMNI (P = 0.0015).
Conclusion: IMPT significantly reduces all cardiac substructure doses regardless of tumor laterality and target volume. Left-sided IMNI and patients with baseline cardiovascular risk factors benefited more from IMPT using ACE as endpoints. More comprehensive cardiac toxicity endpoints need to be developed to better evaluate the value of novel RT techniques in cardiac protection.