 RATIONALE: 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 work in different ways to stop the growth of cancer cells, either by killing the cells or by stopping them from dividing. Radiolabeled monoclonal antibodies, such as yttrium Y 90 ibritumomab tiuxetan, can find cancer cells and carry cancer-killing substances to them without harming normal cells. Giving rituximab together with combination chemotherapy and yttrium Y 90 ibritumomab tiuxetan may kill more cancer cells. 