RecruitingDrug trialPembrolizumab And Stereotactic Radiosurgery (Srs) Of Selected Brain Metastases In Breast Cancer PatientsPatients with metastatic breast cancer with at least 2 brain metastases will receive pembrolizumab every 3 weeks. Patients will undergo stereotactic radiosurgery (SRS) to one of the brain lesions. Pembrolizumab infusion will be given on Day 4 (+/-1) after SRS treatment at the standard dose of 200mg IV over 30 minutes and repeated every 3 weeks until disease progression or unacceptable toxicity.