The purpose of this study is to test the good and bad effects of combining the study drug pembrolizumab with radiation therapy in people whose bladder cancer has shrunk or cannot be found after earlier treatment. The study doctors hope to learn if the study drug controls the cancer so patients don’t need to have their bladder removed.

Pembrolizumab is considered investigational, which means it has not been approved by the U.S. Food and Drug Administration (FDA). Pembrolizumab is approved by the FDA to treat other cancers. 

Study participants will receive 20 radiation therapy treatments and pembrolizumab administered intravenously through a vein in the arm for up to 1 year. Participants will also come into the clinic for study visits every 6 months until 2 years after treatment and then once a year for the remainder of the study until 5 years after treatment.

Total study participation will last approximately 5 years.