A radiolabeled monoclonal antibody that is used to treat certain types of B-cell non-Hodgkin lymphoma and is being studied in the treatment of other types of B-cell tumors. It is made up of the monoclonal antibody ibritumomab plus the radioisotope yttrium Y 90. It binds to the protein called CD20, which is found on B cells. The radiation in the yttrium Y 90 may kill the cancer cells. Yttrium Y 90 ibritumomab tiuxetan is a type of radiopharmaceutical. Also called IDEC-Y2B8, Y 90 ibritumomab tiuxetan, and Y 90 Zevalin.