A Study on Adding Precisely Targeted Radiation Therapy (Stereotactic Body Radiation Therapy) to the Usual Treatment Approach (Drug Therapy) in People With Breast Cancer

Inclusion Criteria: * Age 18 or older * Willing and able to provide informed consent * Metastatic breast cancer, biopsy proven * ER+/HER2-, defined as \>5% ER+ staining * HER2+ (regardless of ER status), including HER2-low and high expressors * History of at least 6 months, sustained response to systemic therapy (clinically or radiographically defined as complete or stable response without progression) * Isolated site of disease progression on FDG PET scan * Consented to 12-245 * ECOG performance status 0-1 Exclusion Criteria: * Pregnancy * Serious medical comorbidity precluding radiation, including connective tissue disorders * Intracranial disease (including previous intracranial involvement) * Previous radiotherapy to the intended treatment site that precludes developing a treatment plan that respects normal tissue tolerances.