Yttrium Y 90 Ibritumomab Tiuxetan and Rituximab in Treating Patients With Previously Untreated Marginal Zone Lymphoma

Inclusion Criteria: * ≥ 18 years old with previously untreated, histologically confirmed Marginal Zone Lymphoma (non-gastric extranodal MZL, splenic MZL and nodal MZL) or gastric MZL that did not respond to antibiotic therapy given up to 6 months prior to enrollment, but not less than 2 months) * Measurable and evaluable disease * All stages are eligible * Eastern Cooperative Oncology Group (ECOG) performance status score of 0, 1, or 2 (Appendix B) * Willing and able to provide written informed consent * Women of childbearing potential must have a negative pregnancy test at study entry and must agree to use effective contraception while on treatment and for 6 months after treatment * Life expectancy of at least 6 months Exclusion Criteria: * Prior chemotherapy, radiation therapy, immunotherapy, systemic corticosteroids, or systemic biologic anticancer therapy before beginning study treatment. * ≥ 25% lymphoma bone marrow involvement * Platelet count \< 100,000 cells/mm³ * Neutrophil count \< 1,500 cells/mm³ * Known history of HIV infection * Pregnant or lactating (pregnancy test is required for all female patients of childbearing potential) * Woman of childbearing potential or sexually active man unwilling to use adequate contraceptive protection. * Physical or mental condition that makes patient unable to complete specified follow-up assessments