Regional Radiotherapy Omission in Low-Risk Node Positive Breast Cancer

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.