Purpose

RATIONALE: Monoclonal antibodies, such as cetuximab, can block tumor growth in different ways. Some find tumor cells and kill them or carry tumor-killing substances to them. Others interfere with the ability of tumor cells to grow and spread. Drugs used in chemotherapy, such as cisplatin and fluorouracil, work in different ways to stop the growth of tumor cells, either by killing the cells or by stopping them from dividing. Radiation therapy uses high-energy x-rays to kill tumor cells. Cetuximab may help cisplatin and fluorouracil work better by making tumor cells more sensitive to the drugs. It may also make tumor cells more sensitive to radiation therapy. Giving cetuximab together with chemotherapy and radiation therapy may kill more tumor cells. PURPOSE: This phase II trial is studying how well giving cetuximab together with cisplatin, fluorouracil, and radiation therapy works in treating immunocompetent patients with stage I (closed to accrual as of 11/3/2008), stage II, (some stage II closed to accrual as of 11/3/2008) or stage III anal cancer.