Breast cancer subjects with multiple brain metastases are often initially treated with stereotactic radiosurgery (SRS), a non-surgical radiation technique used to treat small brain tumors or whole brain radiation therapy (WBRT). When initially managed with SRS, subjects often develop new brain metastases in the untreated brain, requiring additional SRS procedures and, eventually, WBRT, which is often associated with significant neurocognitive sequelae (headache, nausea, dizziness, confusion, memory loss and motor dysfunction). Strategies to decrease the rate of new brain metastases are warranted.

This study will test whether combining SRS with a study immunotherapy drug can elicit an immune response against the irradiated tumor lesion and tumor lesion distal to the irradiated site. The immunotherapy tested in this trial is Pembrolizumab (Keytruda), which blocks a protective mechanism of cancer cells and allows the immune system to destroy those cancer cells. 

In summary, we are proposing to test whether SRS to a select number of brain metastases in combination with Pembrolizumab can effectively induce an immune response in the brain and rest of the body leading to tumor regression