This study evaluates the impact of elective regional lymph node irradiation on N1 breast cancer patients receiving post-lumpectomy radiotherapy and anthracycline plus taxane (AT)-based chemotherapy. We randomly assign patients having one to three metastatic lymph nodes (pN1) after breast-conserving surgery (BCS) and AT-based chemotherapy to undergo either whole-breast and regional nodal irradiation (WB+RNI group) or whole-breast irradiation alone (WBI group).