Additional Study Information: This study is being done to find a safe dose level of REGN2810 alone and in combination with radiotherapy, cyclophosphamide, and radiotherapy plus cyclophosphamide together. Other purposes of this study are to measure the levels of REGN2810 in particpants blood and to collect any evidence of anti-tumor activity of REGN2810 when given alone or in combination with the other therapies. REGN2810 is a fully human monoclonal anti - PD-1 antibody (a type of protein) that works by blocking the programmed death receptor 1 (PD-1), a cell receptor on immune cells that is involved in preventing the immune cells from destroying other cells. This study is being done to test the safety of REGN2810 and to see if blocking PD-1 with REGN2810 will help the immune system attack the tumor.