TAS-102 and Radiation Therapy in Treating Patients With Rectal Cancer That Is Locally Recurrent, Metastatic, or Cannot Be Removed by Surgery

This phase I trial studies the side effects and best dose of trifluridine/tipiracil hydrochloride combination agent TAS-102 (TAS-102) when given together with radiation therapy in treating patients with rectal cancer that has come back, spread to other places in the body, or cannot be removed by surgery. Drugs used in chemotherapy, such as TAS-102, work in different ways to stop the growth of tumor cells, either by killing the cells, by stopping them from dividing, or by stopping them from spreading. Radiation therapy uses high-energy x-rays to kill tumor cells and shrink tumors. Giving TAS-102 with radiation therapy may kill more tumor cells.