Combination Chemotherapy and Bevacizumab With or Without Radiation Therapy in Treating Patients With Locally Advanced Rectal Cancer

RATIONALE: Drugs used in chemotherapy, such as oxaliplatin, 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. Monoclonal antibodies, such as bevacizumab, can block tumor growth in different ways. Some block the ability of tumor cells to grow and spread. Others find tumor cells and help kill them or carry tumor-killing substances to them. Bevacizumab may also stop the growth of rectal cancer by blocking blood flow to the tumor. Radiation therapy uses high-energy x-rays to kill tumor cells. Giving combination chemotherapy and bevacizumab together with radiation therapy before surgery may make the tumor smaller and reduce the amount of normal tissue that needs to be removed. PURPOSE: This clinical trial is studying how well giving combination chemotherapy and bevacizumab with or without radiation therapy works in treating patients with locally advanced rectal cancer