A drug used with the drug rituximab to treat certain types of B-cell non-Hodgkin lymphoma. It is also being studied in the treatment of other types of B-cell tumors. Y 90 ibritumomab tiuxetan contains a monoclonal antibody that binds to a protein called CD20, which is found on B cells. It also contains a radioactive substance called yttrium Y 90, which may help kill cancer cells. Y 90 ibritumomab tiuxetan is a type of radioimmunoconjugate. Also called IDEC-Y2B8, Y 90 Zevalin, and yttrium Y 90 ibritumomab tiuxetan.