Capecitabine, Oxaliplatin, and Radiation Therapy in Treating Patients With Stage II or Stage III Anal Cancer

RATIONALE: Drugs used in chemotherapy, such as capecitabine and oxaliplatin, work in different ways to stop tumor cells from dividing so they stop growing or die. Capecitabine may stop the growth of tumor cells by stopping blood flow to the tumor. Radiation therapy uses high-energy x-rays to damage tumor cells. Capecitabine and oxaliplatin may make tumor cells more sensitive to radiation therapy. Combining capecitabine and oxaliplatin with radiation therapy may kill more tumor cells. PURPOSE: This phase II trial is studying how well giving capecitabine and oxaliplatin together with radiation therapy works in treating patients with stage II or stage III anal cancer.