RecurIndex Guided Avoidance of Regional Nodal Irradiation for 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）Three subtypes are allowed (Lumina A, Lumina B/HER2 negative, Lumina B/HER2 positive) , namely ER positive (ER ≥ 1%), HER2 negative (HER2 expression negative or + by IHC, or 2+ but negative by FISH) or ER positive (ER ≥ 1%), HER2 positive (3+ by IHC, or 2+ but positive 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 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.
* No contralateral breast cancer must be confirmed by molybdenum target and/or MRI within 12 months before randomization of radiotherapy.
* At least 4 courses of adjuvant chemotherapy with anthracycline or taxane should be completed after surgery. If patients with HR positive, HER2 negative tumor underwent Multigene Assays, with low risk scores, omitting chemotherapy is permitted.
* Radiotherapy must be performed sequentially after the completion of adjuvant chemotherapy, starting no later than 8 weeks after the end of chemotherapy.
* The patient must have sufficient primary fresh frozen specimens or post-paraffin tissue sections for RecurIndex testing.
* 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.