Study of Zevalin Versus Observation in Participants at Least 60 Years Old With Newly Diagnosed Diffuse Large B-cell Lymphoma in Positron Emission Tomography (PET)-Negative Complete Remission After Rituximab-Cyclophosphamide, Doxorubicin, Vincristine, and Prednisone (R-CHOP) or R-CHOP-like Therapy

Inclusion Criteria:

1. Participant was 60-years of age or older at time of randomization
2. Histologically confirmed Ann Arbor stage II, III, or IV diffuse large B-cell lymphoma (DLBCL); or follicular lymphoma (FCL) Grade 3B according to the Revised EuropeanAmerican lymphoma (REAL)/ World health organization (WHO) classification (from initialdiagnosis made prior to starting R-CHOP therapy. Results from a pre R-CHOP marrowshall be available for review.
3. Local pathology review confirming the DLBCL diagnosis and cluster of differentiation 20 (CD20) positivity, and no evidence of DLBCL in bone marrow upon confirmation ofcomplete remission (CR).
4. A paraffin block or original slides available for confirmatory pathology review.Participants may be randomized based on the local pathology result.
5. Age-adjusted international prognostic index (IPI) of 1, 2, or 3. The age-adjusted IPIwas defined by one point for Lactate dehydrogenase (LDH) > upper limit of normal (ULN); Stage III or IV; and Karnofsky performance status <80% or WHO/ easterncooperative operations group (ECOG) performance status >1.
6. First-line treatment of DLBCL must have been 6 cycles of standard R-CHOP21, R-CHOP14or dose-adjusted etoposide, prednisone, vincristine, cyclophosphamide, doxorubicin,and rituximab (DA-EPOCH-R) chemotherapy. Participants who received pre-phase therapyfor the purpose of improving performance status prior to initiating R-CHOP areeligible.
7. Complete remission (CR) according to the International Workshop Response Criteria fornon-Hodgkin's lymphoma (NHL) described by Cheson et al after first-line treatment.Computerized tomography (CT) scans of chest, abdomen, pelvis, and neck (if applicable)must have been performed within 6 weeks after the last dose of the last course ofchemotherapy. Applicability of the neck CT means that the participant had involvementof the neck region by palpation / physical examination at first diagnosis.
8. A negative Fluorine-18-deoxyglucose positron emission tomography (FDG-PET) scanconfirming complete response, with negative defined as a score of 1-3 on the Deauville 5-point scale used to quantify radionucleotide density in PET scans as determinedlocally (Morschhauser 200735).
9. Bone marrow cellularity greater than 15%, no evidence of myelodysplasiamorphologically and no evidence of involvement with lymphoma either at the pre R-CHOPmarrow or on repeat assessment pre-Zevalin. After completing R-chemotherapy, a repeatmarrow is required for participant randomized to the Zevalin arm only.
10. A world health organization/eastern cooperative oncology group (WHO/ECOG) performancestatus of 0, 1 or 2.
11. Adequate hematopoietic functions: Absolute neutrophil count (ANC) ≥ 1.0 x 10^9/ liter (L), Hemoglobin (Hgb) ≥ 9 g/dL, Platelets ≥ 100 x 10^9/L.
12. Life expectancy of 6 months or longer.
13. Written informed consent obtained according to local guidelines.

Exclusion Criteria:

1. Presence of any other malignancy or history of prior malignancy within 5 years ofstudy entry. Within 5 years, participants treated for Stage I or II cancers areeligible provided they have a life expectancy of > 5 years. The 5-year exclusion ruledoes not apply to-non melanoma skin tumors and in situ cervical cancer.
2. Prior radioimmunotherapy, including radiation therapy for Non-Hodgkin's lymphoma) NHL,or any other NHL therapy.
3. Presence of primary gastric, central nervous system (CNS), or testicular lymphoma atfirst diagnosis.
4. Histological transformation of low-grade NHL.
5. Active hepatitis B or C.
6. Known history of human immunodeficiency virus (HIV) infection.
7. Abnormal liver function: total bilirubin > 2 × ULN unless secondary to Gilbertdisease.
8. Abnormal renal function: serum creatinine > 2.0 × ULN.
9. Non-recovery from the toxic effects of chemotherapy to < grade 2, or interfering withZevalin treatment.
10. Known hypersensitivity to murine or chimeric antibodies or proteins.
11. Granulocyte-colony stimulating factor (G-CSF) or Granulocyte macrophage-colonystimulating factor (GM-CSF) therapy within 4 weeks prior to Zevalin or observation.
12. Concurrent severe and/or medically uncontrolled disease (e.g. uncontrolled diabetes,congestive heart failure, myocardial infarction within 6 months of study, unstable anduncontrolled hypertension, chronic renal disease, or active uncontrolled infection)which could compromise participation in the study.
13. Treatment with investigational drugs less than 4 weeks prior to Zevalin orobservation.
14. Major surgery less than 4 weeks prior to Zevalin or start of observation.
15. Concurrent systemic corticosteroid use for any reason except as premedication in caseof known or suspected allergies to contrast media or as premedication for potentialside effects of rituximab treatment. Participants on a chronic dose of prednisone fora medical condition (e.g. Asthma or autoimmune disease) less than or equal to 20milligram (mg) daily, stable for 4 weeks, are permissible.
16. Unwillingness or inability to comply with the protocol.