<qa>  What is the name of the drug used in chemotherapy?RATIONALE: Radiation therapy uses high-energy x-rays to kill tumor cells. Drugs used in chemotherapy, such as fluorouracil and leucovorin, 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 and chemotherapy before surgery may make the tumor smaller and reduce the amount of normal tissue that needs to be removed. Giving chemotherapy after surgery may kill any tumor cells that remain after surgery. It is not yet known whether radiation therapy is more effective with or without chemotherapy when given before surgery for rectal cancer. 