This multi-institutional retrospective study was approved by the Korean Radiation Oncology Group (KROG 14-22) and the Institutional Review Board of each of 10 participating institutions in Korea. After obtaining this approval, we reviewed the medical records of patients with breast cancer treated by mastectomy from 2005 to 2010. The eligibility criteria were (1) a tumor size of ≤ 5 cm (pT1 and pT2), (2) negative LNs (pN0) proven by axillary dissection or sentinel LN biopsy, and (3) no treatment with adjuvant PMRT. The exclusion criteria were (1) male gender, (2) the presence of distant metastasis (DM) at diagnosis, (3) neoadjuvant systemic treatment, (4) a history of radiotherapy (RT) to the chest or neck, (5) a history of malignancies other than papillary/follicular thyroid cancer, and (6) bilateral breast cancer. We identified 1,842 patients according to these eligibility criteria. We then excluded 14 patients who were lost to follow-up < 6 months from the mastectomy date. Finally, 1,828 patients with breast cancer were included in this study.