RATIONALE: Drugs used in chemotherapy, such as cyclophosphamide, doxorubicin, vincristine, and prednisone, use different ways to stop cancer cells from dividing so they stop growing or die. Radiation therapy uses high-energy x-rays to damage cancer cells. Monoclonal antibodies, such as rituximab and yttrium Y 90 ibritumomab tiuxetan, can locate cancer cells and either kill them or deliver radioactive cancer-killing substances to them without harming normal cells. Combining chemotherapy with radiation therapy and monoclonal antibody therapy may kill more cancer cells.