Pembrolizumab With or Without Radiation in Patients With Recurrent or Metastatic Adenoid Cystic Carcinoma

Inclusion Criteria: * Participants must have histologically confirmed adenoid cystic carcinoma with evidence of recurrent or metastatic disease not amenable to potentially curative surgery or radiotherapy. * Participants must have at least two RECIST v1.1 measurable non-CNS based lesions. Palliative radiation must be indicated for at least one of these lesions, and this lesion must be a candidate for radiation to a dose of 30 Gy of radiation over 5 fractions as deemed by a treating radiation oncologist. Documentation of volume and prescription isodose line will be collected. Re-irradiation of a previously treated lesion is not allowed. One lesion must be in a location where it will not be incorporated into the radiation fields so systemic response can be assessed. However, inclusion of patients with more than 10 measurable lesions is strongly discouraged and all patients must have life expectancy \> 6 months. * Participants must agree to undergo a research biopsy, if tumor is safely accessible, at baseline and after 2 cycles of pembrolizumab. Participants can be exempt if archival tumor tissue has been collected within 12 months of study enrollment that the Principal Investigator deems it appropriate/sufficient for analysis on this protocol. Biopsy of a lesion outside of the potential radiation treatment field is preferred to maintain consistency across cohorts. * Participants must have archival tissue from the primary tumor or metastases available for correlative studies. E…