Feasibility of omitting regional nodal irradiation in cT1–2N1 breast cancer with ypN1 disease after neoadjuvant chemotherapy (KROG 21-06)

Purpose: To evaluate the effect of regional nodal irradiation (RNI) in breast cancer patients with 1–3 residual nodal metastases (ypN1) after neoadjuvant chemotherapy (NAC). Methods: We retrospectively reviewed the medical records of breast cancer patients with clinical T1–2N1 disease who received NAC followed by surgery and postoperative radiotherapy between 2005 and 2017. Patients with 1–3 pathologically positive lymph nodes (ypN1) after axillary dissection were included. Univariate and multivariate analyses were performed to identify the prognostic factors for locoregional recurrence-free survival (LRRFS), distant metastasis-free survival (DMFS), disease-free survival (DFS), and overall survival (OS). Results: The median follow-up duration was 88.9 months (range, 5.0–200.1). A total of 245 patients were included and RNI was administered in 84.5% (n = 207). Lymphatic invasion and extranodal extension were more frequent in patients who received RNI compared with those who did not (p = 0.009 and p < 0.001, respectively), while other patient and tumor characteristics were similar between the two groups. On multivariate analysis, histologic grade was identified as a significant prognostic factor for LRRFS (p = 0.045) and lymphatic invasion was significantly associated with DMFS (p = 0.029). However, RNI was not associated with any of the survival outcomes. The 5-year LRRFS, DMFS, DFS, and OS in the RNI and non-RNI groups were 89.5% vs. 97.2%, 84.2% vs. 91.6%, 81.7% vs. 91.6% and 91.1% vs. 100%, respectively (all p > 0.05). Conclusion: RNI was not associated with additional survival benefit in cT1–2N1 breast cancer patients with ypN1 disease after NAC. These findings warrant further validation.