Primary Breast Oligoprogressive Sites Treated With Radiotherapy to Obviate the Need to Change Systemic Therapy (BOSS)

Inclusion Criteria: * Age ≥ 18 years. * Histological confirmation of primary breast cancer. * Patients with metastatic breast cancer and at least 12 months of clinical response to first-line systemic therapy, with the subsequent development of 1-3 extracranial sites of oligoprogressive disease, and who plan for the continuation of the current systemic therapy. * NOTE: patients with de novo metastatic disease and those developed metastatic disease after initially localized disease can both be included. OR * Patients with metastatic breast cancer and at least 6 months of clinical response to second-line systemic therapy (or further, e.g., third- or fourth-line), with the subsequent development of 1-3 extracranial sites of oligoprogressive disease. * Eastern Cooperative Oncology Group (ECOG) Performance Status (PS) ≤ 2. * Negative urine or serum pregnancy test done ≤ 7 days prior to registration, for women of childbearing potential only. * Ability to complete questionnaire(s) by themselves or with assistance. * Provide written informed consent. * Willing to return to enrolling institution for follow-up (during the Active Monitoring Phase of the study). * Willing to provide blood samples for correlative research purposes. * Receiving radiation therapy as specified in the protocol. Exclusion Criteria: * Male patients. * Nursing or pregnant women. * Men or women of childbearing potential who are unwilling to employ adequate contraception. * Patients with triple negative disease (negative for ER, PR, and HER2). * Active second primary malignancy * More than 3 extracranial sites of oligoprogressive disease * Active CNS disease. Patients with asymptomatic and stable, treated CNS lesions (radiation and/or surgery and/or other CNS-directed therapy who have not received corticosteroids for at least 4 weeks) are allowed. * Active connective tissue disease that is felt by the treatment team to pose an excess risk of toxicity. * Prior radiation that overlaps with the intended treatment volume such that, in the opinion of the patient's Radiation Oncologist, radiotherapy to progressing sites will not be safe. * NOTE: patients with some dose overlap with prior radiotherapy that is deemed safe by the patient's Radiation Oncologist can be included in the trial.