A monoclonal antibody that is used to treat certain types of B-cell non-Hodgkin lymphoma and is being studied in the treatment and detection of other types of B-cell tumors. Monoclonal antibodies are made in the laboratory and can locate and bind to substances in the body, including cancer cells. Ibritumomab binds to the protein called CD20, which is found on B cells. It is linked to the compound tiuxetan. This allows certain radioisotopes to be attached before it is given to a patient. It is a type of monoclonal antibody-chelator conjugate. Also called Zevalin.