Cardiac Substructure Dosimetry in Postoperative Breast-Conserving Radiotherapy: A Novel 8-Field IMRT Approach for Internal Mammary Node Irradiation Using MONACO

Ziyi Xie¹Shuai Hao²Xiao Wu¹Yinliang Liu¹Chaoen Bao¹Ruhan Zhao¹Ming Liu¹Xiaohui Cao^1*

• ¹Third Hospital of Hebei Medical University, Shijiazhuang, China
• ²Aerospace Center Hospital, Beijing, China

Objective: To evaluate the dosimetric impact of internal mammary lymph node inclusion versus exclusion on cardiac substructures in postoperative breast-conserving radiotherapy. Methods: This study included 20 post-lumpectomy breast cancer patients. The CTV encompassed the ipsilateral breast, supraclavicular lymph nodes, and IMN. Organs at risk comprised the heart and its substructures,as well as bilateral lungs, ipsilateral/contralateral lungs, contralateral breast, thyroid, and spinal cord. For both target delineation strategies, treatment planning utilized 8-field fixed-beam IMRT with 6 MV X-rays. Continuous variables were reported as mean ± standard deviation. Normality was assessed using Shapiro-Wilk tests, with paired t-tests applied for normally distributed data and Wilcoxon signed-rank tests for non-parametric comparisons. Statistical significance was defined as P<0.05 (two-tailed). Results: Left-sided breast cancer analysis: The IMN-irradiated cohort demonstrated significantly elevated cardiac dose parameters, with increased Dmax and Dmean for the whole heart compared to non-IMN treatment. Paradoxically, the left ventricle exhibited reduced mean dose in the IMN group. Significant dose escalation was observed in the right cardiac structures, including right ventricle Dmax/Dmean, right atrium Dmean, anterior myocardial territory Dmax, and right coronary artery Dmax/Dmean. For right-sided breast cancer cases: The IMN-irradiated cohort demonstrated significantly elevated maximum dose (Dmax) to the RA. Myocardial territory: The IMN group showed increased maximum dose and significantly higher V10 to the AMT. Coronary arteries: A paradoxical reduction in LCX Dmax was observed in the IMN-treated patients. Conclusion : 1. The dosimetric analysis reveals that 8-field IMRT with IMN irradiation significantly increases mean heart dose and critical cardiac substructure exposure in left-sided breast cancer (P<0.05), potentially elevating long-term cardiovascular toxicity risks. These findings suggest that clinical decision-making should carefully weigh the competing risks of locoregional recurrence against potential cardiotoxicity, with consideration given to advanced radiotherapy techniques for cardiac dose optimization when treating left-sided malignancies. 2. Comprehensive dosimetric assessment requires delineation of cardiac substructures as discrete organs-at-risk, with their dose constraints systematically integrated into plan optimization and quality assurance protocols. This paradigm shift toward substructure-sparing approaches could enhance the therapeutic ratio by minimizing late cardiovascular complications while maintaining target coverage. 3. Postoperative breast-conserving radiotherapy should follow guideline recommendations regarding IMN irradiation strictly.