Atezolizumab + Stereotactic Radiation in Triple-negative Breast Cancer and Brain Metastasis

Inclusion Criteria: * Participants must have histologically or cytologically confirmed Stage IV invasive breast cancer. Participants without pathologic or cytologic confirmation of metastatic disease should have unequivocal evidence of metastasis from physical examination or radiologic evaluation. * Either the primary tumor and/or metastatic tumor must be triple-negative as defined below: * Hormone receptor status: the invasive tumor must be ER- and PR-negative, or staining present in \<1% by immunohistochemistry (IHC) * HER2 status: the invasive tumor must be Human Epidermal Growth Factor Receptor 2 Negative (HER2-negative) by the ASCO CAP guidelines * In cases where both primary tumor and metastatic sample(s) have been tested for ER, PR, and HER2, the triple-negative status of the most recent sample should be used. * Participants must have a diagnosis of brain metastases for which SRS is indicated, as determined by a radiation oncologist. * The contrast-enhancing intraparenchymal brain metastases(s) must be well circumscribed and must have a maximum diameter of ≤ 3.0 cm in any direction on the enhanced scan. * Participants must not have more than 5 new or progressive lesions in the brain requiring SRS treatment (greater than 5 total brain lesions are allowed as long as no more than 5 lesions require SRS treatment). * Participants must have measurable extracranial disease as defined by RECIST 1.1. * Participants must be willing to undergo a research biopsy at baseline …