Inclusion Criteria:

• B-cell non-Hodgkin’s lymphoma of one of the following types as defined by the WHO classification:
• Small lymphocytic lymphoma/Chronic lymphocytic leukemia
• Nodal marginal zone B-cell lymphoma
• Extranodal marginal zone B-cell lymphoma
• Splenic marginal zone lymphoma
• Lymphoplasmacytic lymphoma
• Follicular lymphoma
• Diffuse large B-cell lymphoma which has transformed from one of the previously listed types of lymphomas
• At least one site of lymphoma greater than or equal to 5 cm in any dimension
• Received at least one prior therapy
• Should show evidence of symptomatology as a result of their disease and/or evidence of progression of their disease.
• Measurable disease using Cheson criteria [23] for Non-Hodgkin’s Lymphoma: a lymph node that is greater than 1 cm in its longest transverse diameter by CT scan should be considered compatible with involvement by NHL
• No anti-cancer therapy for four weeks (six weeks if rituximab, nitrosourea or Mitomycin C) prior to study initiation, and fully recovered from all toxicities associated with prior surgery, radiation treatments, chemotherapy, or immunotherapy
• An IRB-approved signed informed consent
• Age greater than or equal to 18
• Expected survival greater than or equal to 3 months
• Prestudy performance status of 0, 1, or 2 according to WHO
• Absolute neutrophil count ([segmented neutrophils + bands] x total WBC) > 1500/mm3 within two weeks prior to first dose of external beam radiation and also documented again two weeks prior to 90Y-ibritumomab tiuxetan treatment
• Platelet counts greater than or equal to 100,000 within two weeks prior to first dose of external beam radiation and also documented again two weeks prior to 90Y-ibritumomab tiuxetan treatment
• Bone marrow involvement by NHL less than 26% within six weeks of treatment with 90Y-ibritumomab tiuxetan
• Cellularity of bone marrow > 15% within 6 weeks of study
• Female patients who are not pregnant or lactating
• Men and women of reproductive potential who are following accepted birth control methods (as determined by the treating physician, abstinence is an acceptable method)

Exclusion Criteria:

• Prior radioimmunotherapy
• Presence of CNS lymphoma
• Absolute lymphocyte count ≥ 5000
• HIV or AIDS-related lymphoma
• Large pleural effusions or ascites
• Total bilirubin > 2.0 mg/dL
• Serum creatinine > 2.0 mg/dL
• Patients who, in the opinion of their oncology team, have prior external beam radiation therapy to > 25% of active skeletal marrow (either involved field or regional)
• Patients who have received G-CSF or GM-CSF therapy within two weeks or pegfilgrastim within 4 weeks prior to treatment
• Serious nonmalignant disease or infection which, in the opinion of the investigator, would compromise other protocol objectives
• Major surgery, other than diagnostic surgery, within four weeks
• Pregnant women or women of child-bearing age who refuse pregnancy tests
• Patients who have had prior myeloablative autologous or allogenic stem cell transplantation