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Background The internal mammary (IM) lymph node chain, along with the axillary nodal basin, is a first-echelon breast lymphatic draining site. A growing body of evidence supports irradiation of this region in node-positive breast cancer. This study evaluated the effectiveness of radiotherapy in treating magnetic resonance imaging (MRI)-detected abnormal IM lymph nodes in newly-diagnosed non-metastatic breast cancer. Methods A structured query was performed on an electronic institutional database to identify women with radiographic evidence of abnormal IM node(s) on breast MRI from 2005 to 2013. Manual review narrowed inclusion to patients with a primary diagnosis of non-metastatic breast cancer with abnormal IM node(s) based on pathologic size criteria and/or abnormal enhancement. Results Of the 7070 women who underwent pre-treatment MRI, 19 (0.3%) were identified on imaging to have a total of 25 abnormal pre-treatment IM lymph nodes, of which 96% were located in the first two intercostal spaces and 4% in the third space. A majority of the primary tumors were high-grade (94.7%) and hormone-receptor negative (73.7%), while 47.4% overexpressed HER-2/neu receptor. Axillary nodal disease was present in 89.5% of patients, while one patient had supraclavicular involvement. At a median follow-up of 38 months, 31.6% of patients had developed metastatic disease and 21.1% had died from their disease. Of the patients who received IM coverage, none had progressive disease within the IM lymph node chain. Conclusions Radiologic evidence of pre-treatment abnormal IM chain lymph nodes was associated with advanced stage, high grade, and negative estrogen receptor status. The majority of positive lymph nodes were located within the first two intercostal spaces, while none were below the third. Radiation of the IM chain in combination with modern systemic therapy was effective in achieving locoregional control without surgical resection in this cohort of patients.