Intensity-Modulated Radiation Therapy, Fluorouracil, and Mitomycin C in Treating Patients With Invasive Anal Cancer

RATIONALE: Specialized radiation therapy that delivers a high dose of radiation directly to the tumor may kill more tumor cells and cause less damage to normal tissue. Drugs used in chemotherapy, such as fluorouracil and mitomycin C, work in different ways to stop the growth of tumor cells, either by killing the cells or by stopping them from dividing. Giving radiation therapy together with 5-fluorouracil (5-FU) and mitomycin C may kill more tumor cells. PURPOSE: This phase II trial is studying the side effects and how well giving intensity-modulated radiation therapy together with fluorouracil and mitomycin C works in treating patients with invasive anal cancer.