Inclusion Criteria
•Patients must have a histologically confirmed diagnosis of lowgrade non-Hodgkin’s B-cell lymphoma. The following low-grade histologies are included: small lymphocytic (with or without plasmacytoid differentiation); follicular, small cleaved; follicular, mixed small cleaved and follicular large cell (less than 50% large cell component); and monocytoid B-cell lymphoma.
•Patients must have evidence that their tumor tissue expresses the CD20 antigen. Immunoperoxidase stains of paraffin-embedded tissue showing positive reactivity with L26 antibody or immunoperoxidase stains of frozen tissue showing positive reactivity with Anti-B1 Antibody (Coulter Clone®) or similar commercially available CD20 antibody (>50% of tumor cells are positive) or evidence of CD20 positivity by flow cytometry (>50% of tumor cells are positive) are acceptable evidence of CD20 positivity. Testing of tumor tissue from any time in the course of the patient’s disease is acceptable.
•Patients must have Ann Arbor stage III or IV extent of disease after complete staging.
•Patients must not have had any previous treatment for low-grade lymphoma including chemotherapy or radiation. They may be newly diagnosed or observed without treatment after diagnosis. Symptomatic and asymptomatic patients will be eligible.
•Patients must have a performance status of at least 60% on the Karnofsky Scale and an anticipated survival of at least 3 months.
•Patients must have an absolute neutrophil count (ANC) >1500 cells/mm3 and a platelet count >100,000 cells/mm3 within 14 days of study entry. These blood counts must be sustained without support of hematopoietic cytokines or transfusion of blood products.
•Patients must have adequate renal function (defined as serum creatinine <1.5 x upper limit of normal [ULN]) and hepatic function (defined as total bilirubin <1.5 x ULN and aspartate transaminase [AST] <5 x ULN) within 14 days of study entry.
•Patients must have bi-dimensionally measurable disease. At least one lesion must be ≥2 x 2 cm (by computed tomography [CT] scan).
•Patients must be at least 18 years of age.
•Patients must give written informed consent and sign an IRB– approved informed consent form prior to study entry.

Exclusion Criteria
•Patients with more than an average of 25% of the intratrabecular marrow space involved by lymphoma in bone marrow biopsy specimens as assessed microscopically within 42 days of study entry. Bilateral posterior iliac crest core biopsies are required if the percentage of intratrabecular space involved exceeds 10% on a unilateral biopsy. The mean of bilateral biopsies must be no more than 25%.
•Patients with evidence of active infection requiring intravenous (IV) antibiotics at the time of study entry.
•Patients with New York Heart Association class III or IV heart disease or other serious illness that would preclude evaluation.
•Patients with active obstructive hydronephrosis.
•Patients with prior malignancy other than lymphoma, except for adequately treated skin cancer, in situ cervical cancer, or other cancer for which the patient has been disease-free for 5 years.
•Patients with known HIV infection.
•Patients with known brain or leptomeningeal metastases.
•Patients who are pregnant or nursing. Patients of childbearing potential must undergo a pregnancy test within 7 days of study entry and radiolabeled antibody is not to be administered until a negative result is obtained. Males and females must agree to use effective contraception for 6 months following the radioimmunotherapy.
•Patients with previous allergic reactions to iodine. This does not include reacting to IV iodine-containing contrast materials.
•Patients who were previously given any monoclonal or polyclonal antibodies of any non-human species for either diagnostic or therapeutic purposes. This includes engineered chimeric and humanized antibodies.
•Patients who are concurrently receiving either approved or nonapproved (through another protocol) anti-cancer drugs or biologics.