Rituximab, Fludarabine, Cyclophosphamide, and Yttrium Y 90 Ibritumomab Tiuxetan in Treating Patients With Relapsed B-Cell Non-Hodgkin's Lymphoma

DISEASE CHARACTERISTICS: * Histologically confirmed CD20-positive B-cell non-Hodgkin's lymphoma (NHL), including any of the following subtypes: * Indolent NHL, including any of the following: * Follicular * Lymphoplasmacytoid * Marginal zone * Mantle cell NHL * Transformed B-cell NHL * In at least first relapse with an indication for systemic antineoplastic treatment, as defined by the following: * Local or constitutional (B-) symptoms * Hypersplenism due to splenic involvement * Bulky disease (\> 7.5 cm in diameter) * Impending medical problems derived from rapid disease progression within the past 6 months, as defined by an observed or anticipated \> 50% increase in the sum of the areas calculated from multiplying the greatest perpendicular diameters of each lesion * Measurable lesions of lymphoma infiltration * Medically ineligible for high-dose treatment followed by autologous stem cell support * Adequate bone marrow cellularity (\> 15% of marrow area covered by hematopoiesis) * No CNS, leptomeningeal, spinal cord, or testes lymphoma involvement * No lymphoma lesion mandating emergency radiotherapy * No clinical, cytological, cytogenetic, or histopathologic indication of myelodysplastic syndrome * If there is bone marrow infiltration detected prior to chemoimmunotherapy, patient must undergo a repeat bone marrow biopsy prior to planned treatment with radioimmunotherapy to verify the level of bone marrow infiltration is \< 25% PATIENT CHARACTERISTICS: * ECOG performance status 0-2 * Life expectancy ≥ 3 months * Absolute neutrophil count \> 1,500/mm³ * Platelet count \> 150,000/mm³ * Hemoglobin \> 9 g/dL * Creatinine \< 1.5 times upper limit of normal (ULN) * Bilirubin \< 2 times ULN * ALT and AST \< 2 times ULN * Albumin \> 2.5 g/dL * INR \< 1.5 * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective contraception during and for ≥ 12 months after completion of study treatment * No concurrent severe and/or uncontrolled medical disease that would preclude study compliance, including any of the following: * Uncontrolled diabetes * Congestive heart failure * Chronic renal disease * Active uncontrolled infection * No bleeding risks or disorders, including any of the following: * CNS abnormalities suggesting an increased susceptibility for hemorrhage, including recent history of stroke as demonstrated by cranial contrast-enhanced CT scan * Severe arrhythmia or uncontrolled hypertension * Myocardial infarction within the past 6 months * Diabetic retinopathy with history of symptomatic hemorrhage * Known and potentially active gastrointestinal bleeding foci * Concurrent anticoagulant medication that must be continued even with platelet count \< 20,000/mm³ (e.g., following mitral valve replacement, anti-phospholipid syndrome, or recurrent venous thromboembolism) * Other congenital or acquired hemorrhagic diatheses * No ongoing autoimmune hemolytic anemia * No known presence of anti-murine antibody reactivity * No known hypersensitivity to murine or chimeric antibodies or proteins * No known HIV infection * No psychiatric illness that would preclude study requirements * No other malignant disorder within the past 10 years except basal cell carcinoma of the skin or carcinoma in situ of the cervix PRIOR CONCURRENT THERAPY: * See Disease Characteristics * Recovered from prior therapy * No more than 4 prior systemic anti-lymphoma regimens (including single-agent rituximab) * At least 2 months since prior systemic anti-lymphoma treatment (including single-agent rituximab) * No prior radioimmunotherapy * No prior autologous or allogeneic hematopoietic stem cell transplantation * No prior treatment with purine analogues that has not resulted in remission for \> 1 year * No prior anti-CD20 radioimmunoconjugate therapy * More than 5 years since prior radiotherapy to extensive fields covering lymph node regions on both sides of the diaphragm or \> 50% of the spinal column * More than 4 weeks since prior surgery * No concurrent oral anticoagulant therapy