More and more evidence suggests that local radiotherapy can improve the outcomes for patients with oligometastatic disease. The purpose of this study is to assess the impact of radiotherapy, compared with standard systemic therapy alone, on survival, local control and toxicities in patients with extracranial oligometastatic breast cancer. Eligible patients are randomized in a 1:2 ratio between the control arm (standard systemic therapy), and the WLRT arm (standard systemic therapy + radiotherapy). Randomization will be stratified by three factors: visceral metastasis (yes vs.no), number of metastases(≤2 vs. >2), hormone receptor(positive vs. negative). SBRT technique is preferred. During and after radiotherapy, the patients are followed and the efficacy and toxicities of radiotherapy are evaluated.

Inclusion Criteria:

• 18-70 years old
• ECOG performance status 0-2.
• Newly-diagnosed or metachronous extracranial oligometastatic diseases confirmed by pathology or imaging, with or without locoregional disease (biopsy of metastasis is preferred).
• Total number of locoregional and metastatic lesions of 1-5 , the maximum diameter of lesions ≤5 cm and at least one lesion could be evaluated by RECIST1.1.
• Have received or plan to receive systemic therapy.
• All lesions could be safely treated by radiotherapy.
• Life expectancy > 6 months.
• Have adequate organ function.

Exclusion Criteria:

• Have metastases in the central nervous system.
• have indications for palliative radiotherapy to reduce symptoms, such as pain, bleeding, obstruction, and pending fractures caused by the tumor.
• Have moderate/severe liver dysfunction (Child Pugh B or C) from liver metastases, .
• Malignant pleural effusion
• Unable to tolerate radiotherapy due to serious comorbidity
• Have received prior radiotherapy for target area
• Pregnant or lactating women