Clinical Outcomes of Postoperative Radiotherapy with Regional Nodal Irradiation Excluding Internal Mammary Lymph Nodes in Breast Cancer: A Multi-Institutional Retrospective Analysis

Background Postoperative regional nodal irradiation (RNI) is a standard treatment for breast cancer at high risk of regional recurrence; however, the necessity of including the internal mammary node (IMN) region in the radiation field remains unclear. This study aimed to evaluate treatment outcomes in a large cohort of patients who received postoperative radiotherapy with RNI excluding the IMN region. Methods This study included patients with breast cancer who underwent surgery followed by RNI without IMN irradiation between 2007 and 2018. The primary endpoint was disease-free survival (DFS), and the secondary endpoints were overall survival (OS), breast cancer-specific mortality (BCM), distant metastasis-free survival (DMFS), recurrence patterns, and treatment-related adverse events. Results In total, 799 patients were included. The 5-year DFS, OS, BCM, and DMFS rates were 75.9, 88.3, 9.7, and 77.1%, respectively. Worse outcomes were associated with a higher number of positive lymph nodes and estrogen receptor (ER)-negative disease. Medial/central tumor location and younger age were each significantly associated with poorer outcomes, being associated with worse DFS and DMFS. Bone was the most common recurrence site. ER-negative disease, a higher number of positive lymph nodes, medial/central location, and younger age were significant risk factors for recurrence, particularly distant metastasis. IMN recurrence was rare. Conclusions In this cohort, medial/central tumor location, ER-negative disease, and extensive nodal involvement were associated with poorer outcomes, suggesting that these factors may identify patients who can benefit from IMN irradiation. These findings may serve as important reference data when determining the indication for IMN irradiation on an individual patient basis.