 Treatment in the PLUMMB trial will start with a pembrolizumab 2 weeks prior to starting a course of 6 weeks radiotherapy. Treatment with pembrolizumab will then be given every three weeks. Patients in Group A will then continue to take pembrolizumab for up to a year unless they have disease progression or unacceptable side effects in the meantime. Patients in Group B will continue taking pembrolizumab for as long as needed until they have disease progression or unacceptable side effects. 