Radiation Therapy, Mitomycin, and Either Fluorouracil or Cisplatin in Treating Patients With Locally Advanced Anal Cancer

RATIONALE: Drugs used in chemotherapy, such as mitomycin, fluorouracil, and cisplatin, use different ways to stop tumor cells from dividing so they stop growing or die. Combining radiation therapy with chemotherapy may kill more tumor cells. It is not yet known whether radiation therapy and mitomycin are more effective when combined with fluorouracil or with cisplatin in treating anal cancer . PURPOSE: This randomized phase II/III trial is studying how well giving radiation therapy and mitomycin together with fluorouracil works compared to radiation therapy, mitomycin, and cisplatin in treating patients with locally advanced anal cancer.