After breast-conserving surgery, the role of radiotherapy is reflected in the effect on local control of tumor and long-term survival of patients. The effect of radiotherapy on local control of tumor after breast-conserving surgery has been clearly established, i.e., postoperative radiotherapy can reduce the local recurrence rate of tumor by about 70%, and postoperative radiotherapy not only improves the local control rate of tumor, but also improves the long-term survival rate of patients. MA.20 Clinical studies have shown that regional lymph node radiotherapy after breast-conserving surgery for breast cancer can reduce the risk of local and distant recurrence and prolong patients’ disease-free survival. The study enrolled 1,832 breast cancer patients, 85% of whom had 1-3 positive axillary lymph nodes and 10% of whom were at high risk with negative lymph nodes. All patients received breast-conserving surgery and postoperative chemotherapy or endocrine therapy, and patients were randomized to receive either whole-breast radiotherapy or whole-breast radiotherapy + regional lymph node radiotherapy. The median follow-up of 62 months showed a significant benefit in the group receiving regional lymph node radiotherapy, with a significant improvement in DFS. This clinical study recommended internal breast lymph node radiotherapy, but given that the results of the MA.20 clinical study have not been formally published, its changes in clinical practice await detailed analysis of this clinical study. Patients with older, early-stage and good-prognosis breast cancer still require postoperative radiotherapy. Radiotherapy has always played an important role in breast-conserving treatment and has been refined over the last 30 years, but there are still many aspects that need to be further investigated.