Stereotactic Radiosurgery and Immunotherapy (Pembrolizumab) for the Treatment of Recurrent Meningioma

Inclusion Criteria: * Patients must have histologically confirmed World Health Organization (WHO) grade II or III meningioma that is progressive or with one or more recurrences following surgical resection and radiotherapy * Patients must be eligible/appropriate for treatment with radiation therapy, specifically, if radiation is given in a single session, the target volume cannot exceed 8 ccs or if given in five (consecutive business days) sessions, the target volume cannot exceed 20 ccs * Prior therapy: * There is no limit on the number of prior surgeries or systemically administered therapeutic agents * An interval of \>= 28 days and full recovery (no ongoing safety issues) from surgical resection * An interval of \> 7 days from stereotactic biopsy * For prior systemic agents, participants must be at least 4 weeks (or 5 half-lives, whichever is shorter) from other prior cytotoxic chemotherapy (6 weeks from nitrosoureas) or biologic therapies * For prior radiotherapy, there are no exclusions on number of courses or prior type of radiotherapy. All modalities including prior fractionated external beam photon/proton radiotherapy, stereotactic radiosurgery, and/or or brachytherapy are permissible with a required interval of 6 months from last prior radiotherapy treatment, unless radiation given outside of the planned field, then within 2 weeks from prior radiotherapy treatment. Patients must have had one prior course of radiation therapy * Participants must have reco…