Phase II PEMBROLIZUMAB + PALLIATIVE RADIOTHERAPY IN BC

This research study is a Phase II clinical trial. Phase II clinical trials test the safety and effectiveness of an investigational intervention to learn whether the intervention works in treating a specific disease. "Investigational" means that the intervention is being studied. In this research study, the investigators are evaluating the safety and effectiveness of palliative radiotherapy ("radiation therapy") in combination with pembrolizumab in HR-positive, HER2-negative breast cancer. This study is designed to test how well radiation therapy in combination with pembrolizumab treats this type of cancer. The FDA has approved radiation therapy as a treatment option for this type of breast cancer. The FDA (the U.S. Food and Drug Administration) has not approved pembrolizumab for this specific disease but it has been approved in the United Sates for other types of cancer. Pembrolizumab is a drug that may treat cancer by working with the immune system. The immune system is the body's natural defense against disease. The immune system sends types of cells called "T cells" throughout the body to detect and fight infections and diseases, including cancer. For some types of cancer, the T cells do not work as they should and are prevented from attacking the tumors. Pembrolizumab is thought to work by blocking a protein in the T cells called PD-1 ("programmed death 1"), which then allows these cells and other parts of the immune system to attack tumors. The combination of pembrolizumab and radiation therapy is investigational. The study drug and radiation therapy, when given separately, work in different waysto help stop the cancer cells from growing and spreading. However, it is not known if giving the study drug and radiation therapy at the same time will have a better anti-cancer effect than giving each treatment on its own.