This study is a multicenter, randomized, phase 3 clinical trial in patients with breast cancer, randomizing radiotherapy group (postmastectomy radiation therapy (PMRT)/whole breast irradiation plus regional radiotherapy (WBI+regional RT) versus and no PMRT/WBI alone group. This is a non-inferiority study aiming that there is no significant difference in the 7-year disease-free survival rate between the two groups.

1:1 randomization to the control group and experimental group, stratified by the type of surgery (breast-conserving surgery (BCS) versus mastectomy) and the type of histologic type (triple negative breast cancer (TNBC) versus non-TNBC)

The control group:

• If patients received BCS, WBI+Regional RT
• If patients received mastectomy, PMRT

The experimental group:

• If patients received breast conserving surgery, WBI alone
• If patients received mastectomy, No PMRT

Inclusion Criteria:

• Patients aged 19 years or older.

• A female patient who underwent breast-conserving surgery or mastectomy for invasive breast cancer.

• Patient with stage pN1 after surgery on histopathologic examination.

  • However, if mastectomy was performed, 5 or more nodes should be resected in the case of 1 positive node, and axillary lymph node dissection (ALND) should be performed in case of 2 or 3 positive nodes.

• Patients undergoing or planning to undergo hormone therapy in the case of hormone receptor positive.

• Patients who have received or are expected to undergo targeted therapy in the case of human epidermal growth factor receptor(HER)-2 positive.

• Patients with Eastern Cooperative Oncology Group (ECOG) Scale 0-2.

• Patients who agreed to participate in the study.

Exclusion Criteria:

• Patients who have received prior [neoadjuvant] chemotherapy.
• Patients receiving radiation therapy for salvage or palliative purposes.
• Patients with stage T4.
• Patients with ipsilateral supraclavicular and internal mammary lymph node metastases or distant metastases.
• Male breast cancer patient.
• Patients who have previously received radiation therapy to the ipsilateral breast or supraclavicular region.
• Patients having a history of cancer other than thyroid cancer, cervical carcinoma in situ, or skin cancer.
• Patients diagnosed with ductal breast carcinoma in situ, lobular carcinoma in situ, phyllodes, metaplastic cancer, or benign tumors based on histological diagnosis.