Mantle cell lymphoma cell express a protein called CD20. Ibritumomab tiuxetan is an antibody targeted against CD20, which carries a radioactive material called 90-Yttrium. The radioactivity will be delivered to the cancer cells by the antibody will help killing the mantle cell lymphoma cells. Before treatment starts, patients will have a physical exam, including blood and urine test. Patients will have a chest x-ray and CT scans of the abdomen and pelvis. Bone marrow samples will be taken with a large needle. A cardiogram (EKG) will be performed before therapy and after 1 and 3 months of therapy.