RATIONALE: Radiolabeled monoclonal antibodies, such as iodine I 131 tositumomab, can find cancer cells and carry cancer-killing substances to them without harming normal cells. Monoclonal antibodies, such as rituximab, 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 carry cancer-killing substances to them. Drugs used in chemotherapy, such as cyclophosphamide, doxorubicin, vincristine, and prednisone, work in different ways to stop the growth of cancer cells, either by killing the cells or by stopping them from dividing. Giving a radiolabeled monoclonal antibody together with rituximab and combination chemotherapy may kill more cancer cells. PURPOSE: This phase II trial is studying how well giving iodine I 131 tositumomab together with rituximab and combination chemotherapy works in treating older patients with stage II, stage III, or stage IV B-cell non-Hodgkin's lymphoma.