<qa>  What is the best dose of bevacizumab?This phase I trial is studying the side effects and best dose of bevacizumab when given together with fluorouracil and external-beam radiation therapy in treating patients with stage II or stage III rectal cancer. Monoclonal antibodies, such as bevacizumab, can block cancer growth in different ways. Some block the ability of cancer cells to grow and spread. Others find cancer cells and help kill them or deliver cancer-killing substances to them. Drugs used in chemotherapy 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. Combining monoclonal antibody therapy with chemotherapy and radiation therapy may kill more tumor cells. 