Pembro+Chemo in Brain Mets

Inclusion Criteria: * Non-small cell lunch cancer (NSLC) with untreated asymptomatic brain metastases * NSLC lacks oncogenic driver mutations * Absence of new onset neurological symptoms * Presence of fewer than ten intracranial lesions * Each lesion measures three centimeters or less * Life expectancy of greater than three months * Adequate organ and marrow function * Ability to understand and willingness to sign a written informed consent document Exclusion Criteria: * Presence of oncogenic driver mutations * Measurable lesion located within 10mm of the optic chiasm or optic nerve, or within the brainstem * Known leptomeningeal involvement. * Midline shift * Serious non-healing wound, ulcer or bone fracture * Baseline inability to participate or complete neurocognitive testing * Major surgical procedure (including craniotomy and open brain biopsy) or significant traumatic injury within 14 days prior to registration * Receipt of a non-CNS minor surgical procedure (e.g. core biopsy or fine needle aspiration) within three days prior to registration * History of allergic reactions attributed to monoclonal antibodies (mAb), compounds of similar chemical or biologic composition to Pembrolizumab * Clinically significant cardiovascular disease * Patients with uncontrolled intercurrent illness * Patients with psychiatric illness/social situations that would limit compliance with study requirements