RATIONALE: 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. Drugs used in chemotherapy, such as prednisone, cyclophosphamide, doxorubicin, and vincristine, use different ways to stop cancer cells from dividing so they stop growing or die. Combining monoclonal antibody therapy with chemotherapy may kill more cancer cells.