The purpose of this study is to determine whether certain patients can safely omit regional lymph node radiotherapy. These women must have hormone-sensitive, Her2-negative tumors, have 1-3 positive axillary nodes and have RecurIndex low-risk.

Inclusion Criteria:

• Age ≥ 18 years old, ≤ 70 years old.

• Eastern Cooperative Oncology Group (ECOG) ≤ 2.

• Postoperative pathology is clearly diagnosed as invasive breast cancer. Meet the clinical definition of low risk: ①Axillary lymph node micrometastasis (N1mic) or ②N1 patients who meet the following conditions at the same time,

  a）Age ≥ 40 years old, b）Vascular tumor thrombus (LVI) negative or allow few positive LVI (excluding extensive or massive LVI), c）ER positive (ER ≥ 1%) and HER2 negative (HER2 expression negative or + by IHC, or 2+ but negative by FISH).

• Postoperative pathological diagnosis of axillary lymph node status is any of the following: a. Sentinel lymph node biopsy or axillary lymph node dissection for micrometastasis (N1mic), b. Sentinel lymph node biopsy for 1-2 lymph node macrometastasis (N1sln), c. Sentinel lymph node biopsy + axillary lymph node dissection or simple axillary lymph node dissection for 1-3 lymph node metastasis (N1).

• The patient must have sufficient primary fresh frozen specimens or post- paraffin tissue sections for RecurIndex testing.

• Patients must have RecurIndex low risk obtained from testing of breast tumor tissue from a core biopsy or from the surgical specimen.

• The primary tumor and breast undergo breast-conserving surgery or total resection ± breast reconstruction (autologous/prosthetic).

• There must be adequate systemic examination (such as chest X-ray, B- ultrasound, CT, etc.) within 3 months before randomization of radiotherapy to confirm that there is no distant metastasis.

• Preoperative or radiotherapy should be performed within 12 months of randomization and must have mammography and/or MRI to confirm that there is no contralateral breast cancer.

• At least 4 courses of adjuvant chemotherapy with anthracycline or taxane should be completed after surgery.

• Radiotherapy must be performed sequentially after the completion of adjuvant chemotherapy, starting no later than 8 weeks after the end of chemotherapy.

• No previous history of malignant tumors, except for basal cell carcinoma of the skin.

• Signed an informed consent form.

Exclusion Criteria:

• Postoperative radiotherapy was confirmed as T3-4, N0, N2-3, M1 lesion staging before enrollment.
• Receive any new adjuvant therapy before surgery, including chemotherapy, endocrine therapy, targeted therapy or radiation therapy.
• Patients who underwent total mastectomy and only sentinel lymph node biopsy.
• Have a history of contralateral breast cancer.
• History of chest radiotherapy.
• combined with severe heart, lung, liver, kidney, hematopoietic, neurological diseases, and mental illness.
• History of autoimmune diseases such as scleroderma or active lupus erythematosus.
• pregnant and lactating patients.

pT1-2N1M0 breast cancer