 SECOND PHASE: Patients who achieve a complete response, stop study treatment, and then experience radiographic disease progression may be eligible for the second phase at the discretion of the investigator if no cancer treatment was administered since the last dose of pembrolizumab and trial eligibility safety parameters are met. Patients receive pembrolizumab IV over 30 minutes on day 1. Courses repeat every 3 weeks for up to 17 courses in the absence of disease progression or unacceptable toxicity. 