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