 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, such as yttrium Y 90 ibritumomab tiuxetan, find cancer cells and help kill them or carry cancer-killing substances to them without harming normal cells. Giving rituximab together with yttrium Y 90 ibritumomab tiuxetan may kill more cancer cells. 