RATIONALE: Monoclonal antibodies, such as rituximab, can block cancer growth in different ways. Some find cancer cells and kill them or carry cancer-killing substances to them. Others interfere with the ability of cancer cells to grow and spread. Drugs used in chemotherapy, such as cyclophosphamide, doxorubicin hydrochloride, 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. Radiation therapy uses high-energy x-rays to kill cancer cells. Giving rituximab and combination chemotherapy together with radiation therapy may kill more cancer cells. It is not yet known which schedule of rituximab and combination chemotherapy is more effective when given with or without radiation therapy in treating non-Hodgkin's lymphoma.