| Summary: | Purpose: This study was aimed to assess the outcome of radiotherapy and determine prognostic factors for survival in breast cancer patients with clinically overt metastasis to the internal mammary lymph node (IMN+). Methods: We retrospectively reviewed the medical records of 193 patients with IMN + breast cancer who received neoadjuvant chemotherapy (NAC), breast surgery without internal mammary lymph node (IMN) dissection, and postoperative radiotherapy at 9 hospitals between 2009 and 2013. Breast-conserving surgery or mastectomy was performed after taxane-based NAC. Radiotherapy was administered to the whole breast/chest wall and regional nodes. IMN-covering radiotherapy was performed in 92.2% of patients with median dose of 58.4 Gy (range, 44.9–69.1 Gy). The overall survival (OS), disease-free survival (DFS), and IMN failure-free survival (IMNFFS) were analyzed. Results: After median follow-up of 71 months, 9 patients (4.7%) developed IMN failure and simultaneous distant metastasis. The 5-year DFS, OS, and IMNFFS was 68.6%, 81.8%, and 95.3%, respectively. Non-triple-negative breast cancer, Ki-67 ≤ 10%, pathological complete response (CR) in tumor and axillary node, and radiologic CR of IMN after NAC were significant factors for predicting higher DFS; however, IMN radiation dose was not significant determinants for DFS. The 5-year DFS of patients with IMN-dose ≤ 50.0 Gy and those with >50.0 Gy was 86.7% and 76.7%, respectively (p = 0.41). Conclusions: A multimodality strategy including NAC, breast surgery, and IMN-covering radiotherapy was effective for patients with overt IMN + breast cancer. Even without an IMN dissection, most patients were IMN failure-free with an IMN-focusing radiotherapy.
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