The role of postoperative regional nodal irradiation (RNI) for T1-2N1 intermediate-risk breast cancer is controversial, and there is a lack of class I evidence of a survival benefit from RNI. A number of retrospective studies of breast cancer patient been undertaken to risk stratify, analyse site of recurrence and assess the role of radiotherapy. RNI is currently recommended for patients at high risk of recurrence, but prospective studies in other patients are needed to assess the role of RNI. Two randomised phase III trials have confirmed the safety and efficacy of 15-fraction hypofractionated radiotherapy, while 5-fraction super-hypofractionated radiotherapy to further shorten the course of radiotherapy is a hotspot of current research. The present study aimed to investigate whether RNI improves the outcome of intermediate-risk breast cancer patients; and to assess the efficacy and toxicity of 15-fraction hypofractionated and 5-fraction super-hypofractionated radiotherapy.

Inclusion Criteria:

1. ≥45 years old
2. Receiving breast-conserving surgery or total mastectomy and axillary lymph node dissection or sentinel lymph node biopsy
3. Negative surgical margins
4. Tumor staging： A. patients without neoadjuvant chemotherapy: pT1-2N1M0, axillary lymph node dissection with 1-3 metastatic lymph nodes, at least 1 lymph node with macroscopic metastasis, and a score of 0-2 according to the following risk factors: ≤40 years of age (1 point), tumour located in the inner quadrant (1 point), 2-3 lymph nodes positive in the axilla (1 point), presence of choroidal tumour thrombus (1 point), and restaging according to the 8th edition of the staging criteria (IB-IIA 1 point, and IIB-IIIA 2 points) B. patients without neoadjuvant chemotherapy: pT1-2N1M0, sentinel lymph node biopsy with 1-2 metastatic lymph nodes, at least 1 lymph node with macroscopic metastasis, and a score of 0-2 according to the following risk factors: ≤40 years of age (1 point), tumour located in the inner quadrant (1 point), 2-3 lymph nodes positive in the axilla (1 point), presence of choroidal tumour thrombus (1 point), and restaging according to the 8th edition of the staging criteria (IB-IIA 1 point, and IIB-IIIA 2 points) C. patients with neoadjuvant chemotherapy: cT1-2N1-2M0→ypT0-2N0M0, requiring positive clinical lymph nodes with pathological confirmation, receiving ≥6 cycles of neoadjuvant chemotherapy, and ≥3 sentinel lymph nodes detected for those with simple sentinel lymph node biopsy
5. Signed informed consent

Exclusion Criteria:

1. Distant metastases
2. Metastasis to ipsilateral internal breast, supraclavicular or subclavicular lymph nodes
3. Previous radiotherapy to the chest
4. Bilateral breast cancer
5. Pregnancy, breastfeeding
6. Previous or concurrent other malignancy with tumor-free survival <5 years (but excluding skin cancer with non-malignant melanoma, papillary/follicular thyroid cancer, cervical carcinoma in situ)

IPD often includes sensitive personal information about patients. Sharing such data could breach patient confidentiality agreements.