Outcomes of Patients With Breast Cancer Treated With or Without Internal Mammary Irradiation: A Single-Center, Retrospective Propensity Score–Matched Study

Purpose: Internal mammary lymph node radiation therapy (IMN-RT) has unclear benefits. Historical data were based on only conventional (2-dimensional) radiation techniques. In this 3-dimensional radiation therapy era, we compared the distant metastasis–free survival (DMFS) rates of patients receiving IMN-RT with those who did not include coverage of the IMN (non–IMN-RT). This study aimed to determine the relationship between IMN-RT and distant metastasis control in patients with lymph node–positive breast cancer. Methods and Materials: This was a single-center retrospective cohort study. Patients were divided into 2 groups: IMN-RT and non–IMN-RT. The criterion of the IMN-RT group was that 80% of the prescribed dose covered ≥98% of the Clinical Target Volume of IMN. The primary outcome was 4-year DMFS, and the secondary outcomes were 4-year overall survival, 4-year disease-free survival, and cardiac toxicity. Results: From January 2012 to December 2018, 570 patients were evaluated (IMN-RT, 143 patients; non–IMN-RT, 427 patients). Propensity score matching decreased the number of patients in each group to 139. The median follow-up was 4.3 years. The 4-year DMFS rates were as follows: IMN-RT, 79.1% (95% confidence interval [CI], 70.1%-85.6%), and non–IMN-RT, 82.8% (95% CI, 74.2%-88.7%; P =.43). The groups’ 4-year overall survival and disease-free survival rates did not differ. The 4-year overall survival rates were 84.3% (95% CI, 76.4%-89.8%) for IMN-RT and 88.1% (95% CI, 81.0%-92.7%; P =.39) for non–IMN-RT. The 4-year disease-free survival rates were 77.1% (95% CI, 68.1%-83.8%) for IMN-RT and 82.1% (95% CI, 73.6%-88.1%; P =.29) for non–IMN-RT. There was no significant difference in cardiac toxicity (IMN-RT, 1.4%; non–IMN-RT, 1.4%; P = 1.0). Conclusions: In the modern radiation technique era with real-world data, we could not find a benefit of internal mammary irradiation.