Objective:
* Compare the effect of post-BCS WBI vs. WB+RNI on disease-free survival in pN1 breast cancer patients who received AT-based chemotherapy.
* Evaluate the impact of WBI or WB+RNI according to molecular subtype of tumor.
* Compare the treatment-related toxicities between the WBI and WB+RNI.
* Compare the patient's quality of life between the WBI and WB+RNI.
Outline: This is a a randomized, multi-center, phase III study. Patients are stratified according to molecular subtype of tumor (luminal A vs luminal vs luminal HER2 vs HER2-enriched vs basal-like), axillary lymph node management (sentinel lymph node biopsy \[SLN\] only vs axillary lymph node dissection+/- SLN) and participating institutions. Patients are randomized to one of two treatment arms.
* Arm I: Patients receive WB+RNI.
* Arm II: Patients receive WBI alone.
Arm circumference and quality of life (EORTC QLQ-C30 \& EORTC QLQ-BR23) are assessed within 2 weeks prior to randomization, during the last week of radiotherapy, at 3 months after completion of radiotherapy, and then annually thereafter for five years.
Patients are followed at 3 months after completion of radiotherapy and annually thereafter for seven years.
