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

The purpose of this study is to see if using Stereotactic Body Radiation Therapy/SBRT to treat a single metastatic site where cancer has worsened may be an effective treatment for people with oligometastatic breast cancer. Participants will stay on their usual drug therapy while they receive SBRT. This combination of SBRT to a single metastatic site and usual drug therapy may prevent participants' cancer from worsening in other metastatic sites or spreading.

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.