 Each participant will undergo screening and then be treated with reirradiation with 1.2 Gy BID, 5 days a week weeks 1-5. MK-3475 generic name: pembrolizumab, trade name Keytruda will be given at 200mg intravenous every 3 weeks starting day one of reirradiation and will be continued in all participants until 3 months post completion of reirradiation, at which time a PETCT will be done to evaluate response. Participants with progressive disease PD will be taken off MK-3475 and followed for survival. Participants that have a complete response CR will be followed clinically and radiographically, and if disease recurs may be eligible to be retreated with MK-3475 for up to one year. Participants with partial response PR or stable disease SD will continue treatment with MK-3475 for up to two years unless one of the following occurs: 