Radiation Therapy and Either Capecitabine or Fluorouracil With or Without Oxaliplatin Before Surgery in Treating Patients With Resectable Rectal Cancer

RATIONALE: Drugs used in chemotherapy, such as capecitabine, fluorouracil, and oxaliplatin work in different ways to stop tumor cells from dividing so they stop growing or die. Radiation therapy uses high-energy x-rays to damage tumor cells.
PURPOSE: This randomized phase III trial is studying radiation therapy and either capecitabine or fluorouracil with or without oxaliplatin and comparing them to see how well they work when given before surgery in treating patients with resectable rectal cancer. It is not yet known whether radiation therapy and either capecitabine or fluorouracil is more effective with or without oxaliplatin in treating rectal cancer.