Study of Palliative Radiation Combined with Pembrolizumab in Unresectable Metastatic Stage IV Breast Cancer

This study will enroll subjects who are female aged \> 18 years at the time of informed consent with histologically confirmed diagnosis of breast cancer (unresectable or metastatic stage IV breast cancer). Subjects must have measurable disease and be a candidate for palliative stereotactic body radiation therapy (SBRT). Subjects must have Eastern Cooperative Oncology Group Performance Status (ECOG PS) of 0, 1, or 2 and adequate hematologic, hepatic, renal, and coagulation function. Subject must also have received two prior lines of FDA-approved therapy. Triple negative patients must have had prior exposure to taxane therapy either in the neoadjuvant/adjuvant or metastatic setting. ER + positive patients must have had prior cyclin dependent kinase (CDK) 4/6 inhibitor therapy. Her2 overexpressed patients must have had prior therapy with trastuzumab and ado-trastuzumab (TDM-1).