We have previously demonstrated that gene deletion of the immunosuppressive enzyme IDO1 (indoleamine-2,3-dioxygenase 1) reduces but does not eliminate tumor burden in a mouse model of colitis associated cancer (CAC). In the current study, we test the hypothesis that a new highly specific IDO1 inhibitor epacadostat (Incyte Corp.) will synergize with cytotoxic radiation therapy (RT) as a novel treatment strategy for CAC.