Recent advances in systemic therapy have facilitated improved progression-free survival (PFS) and treatment tolerability in metastatic breast cancer patients (MBC). Oligoprogression (OP) refers to progression limited to five or fewer sites in otherwise controlled systemic disease on a drug therapy. Stereotactic body radiotherapy (SBRT) has the potential to locally ablate resistant OP lesions that develop on a systemic treatment, and may consequently delay the need for change in drug therapy, delay time to chemotherapy and prolong PFS. This is a phase II trial of SBRT plus continuation of current systemic therapy line for OP MBC patients, to determine rate of delay of change in systemic therapy of at six months. PFS, time to chemotherapy and quality of life will also be assessed.

Inclusion Criteria:

1. Diagnosis of progressive metastatic breast cancer on first line systemic therapy, including either hormone receptor positive, Her-2 negative (HR+/Her2-) on endocrine therapy + CDK4/6 inhibitor or Her-2 positive (hormone receptor positive or negative /Her2+) on Her2-targeted therapy regimens.
2. Progressive disease limited to "oligoprogression", defined as progression of 5 or fewer extra-cranial lesions with otherwise controlled systemic disease on current line of systemic therapy.
3. Patients must have previously controlled disease for at least six months on current systemic therapy
4. Deemed a candidate for stereotactic body radiotherapy (SBRT) to all OP lesions

Exclusion Criteria:

1. Requires change in systemic therapy line at the time of OP as determined by medical oncologist;
2. Progression on 2nd line or subsequent lines of therapy
3. Lacks CT or bone scan Imaging within previous 45 days;
4. Progression in >3 sites in the liver or lung;
5. Hormone positive disease on endocrine therapy only at time of enrollment;
6. Previous radiotherapy to same site or vicinity preventing definitive SBRT (e.g. within 5 cm);
7. Lesions deemed not amenable to SBRT due to large size or location;
8. Unacceptable fracture risk according to clinician judgement for bone lesions;
9. Brain metastasis or Spinal cord compression;
10. History of major radiosensitivity syndrome or contraindications to radiotherapy.