Capecitabine, Oxaliplatin, Selenomethionine, and Radiation Therapy in Treating Patients Undergoing Surgery For Newly Diagnosed Stage II or III Rectal Adenocarcinoma

RATIONALE: Drugs used in chemotherapy, such as capecitabine and oxaliplatin, work in different ways to stop the growth of tumor cells, either by killing the cells or by stopping them from dividing. Selenomethionine may slow the growth of tumor cells. Radiation therapy uses high-energy x-rays to kill tumor cells. Giving combination chemotherapy together With selenomethionine and radiation therapy may kill more tumor cells.
PURPOSE: This phase II trial is studying how well selenomethionine works when given together with capecitabine, oxaliplatin, and radiation therapy in treating patients undergoing surgery for newly diagnosed stage II or stage III rectal cancer.