Atezolizumab + Pertuzumab + Trastuzumab In CNS Mets In BC

Inclusion Criteria: \- Eligibility will be assessed as part of the screening procedures for all patients. * Pathologically confirmed HER2-positive MBC by local laboratory with the following requirements: HER2 overexpressed or amplified (immunohistochemistry of 3+ or HER2 gene amplification by in situ hybridization with a ratio of HER2-gene signals to centromere 17 signals ≥ 2.0 or average HER2 copy number ≥ 6.0 signals/cells). * At least one measurable CNS metastasis, defined as ≥ 10 mm in at least one dimension * Unequivocal evidence of new and/or progressive brain metastases, and at least one of the following scenarios: * Treated with SRS or surgery with residual un-treated lesions remaining. Such participants are eligible for immediate enrollment on this study providing that at least one untreated lesion is measurable * Participants who have had prior WBRT and/or SRS and then whose lesions have subsequently progressed are also eligible. In this case, lesions which have been treated with SRS may be considered as target lesions if there is unequivocal evidence, in the opinion of the treating physician, of progression following SRS. * Participants who have not previously been treated with cranial radiation (e.g., WBRT or SRS) are eligible to enter the study, but such participants must be asymptomatic from their CNS metastases and not requiring corticosteroids for symptom control. * Both participants who present with systemic stable/absent or progressive disease are…