Radioimmunotherapy With 90Y-ibritumomab Tiuxetan as Part of a Dose Reduced Conditioning Regimen for Allogeneic Hematopoietic Cell Transplantation for the Treatment of Non-Hodgkin Lymphoma

Inclusion Criteria: * Patients with advanced CD20+ NHL (non-Hodgkin lymphoma) relapsed after at least two preceding chemotherapy regimens including treatment with rituximab or with relapse after autologous HCT (hematopoietic cell transplantation) The following entities of lymphomas can be included in Arm A of the protocol: Arm A: * Small lymphocytic lymphoma (SLL/CLL) * Mantle cell lymphoma (MCL) * Follicular lymphoma Grade 1-2 * Marginal zone lymphoma (MZL) * Extranodal (MALT lymphoma) * Nodal (Monocytoid B-cell lymphoma) * Splenic The following lymphoma entities can be included in Arm B of the protocol: Arm B: * Diffuse large B-cell lymphoma/follicular lymphoma grade 3 * Grade 3 follicular lymphoma * Blastic mantle cell lymphoma * Mediastinal B-cell lymphoma * Age \> 18, \< 70 years * Karnofsky score \> 60% * HLA-identical related or unrelated donor * CD20+ lymphoma cells on biopsy or peripheral blood * Disease stage at inclusion: CR, PR or SD Exclusion Criteria: * Patients with rapidly progressive disease * Less than 3 months after preceding HCT * CNS involvement with disease * Fungal infections with radiological progression after receipt of amphotericin B or active triazole for greater than 1 month * Liver function abnormalities with bilirubin \> 2 mg/dL and elevation of transaminases higher than 2x upper limit of normal * Chronic active viral hepatitis * Ejection fraction \< 40% on echocardiography * Patients with \> grade II hypertension by CTC criteria * Creatinine clearance \< 50 ml/min * Respiratory failure necessitating supplemental oxygen or DLCO \< 30% * Allergy against murine antibodies * HIV infection * Female patients who are pregnant or breast feeding, or adults of reproductive potential not employing an effective method of birth control during study treatment and for at least 12 months thereafter. (Women of childbearing potential must have a negative serum pregnancy test at study entry.) * Patients with pleural effusion or ascites * Concurrent severe and/or uncontrolled medical disease (e.g. uncontrolled diabetes, congestive heart failure, myocardial infarction within 6 months prior to the study, unstable and uncontrolled hypertension, chronic renal disease, or active uncontrolled infection) which could compromise participation in the study * Patients who received any investigational drugs less than 4 weeks before entry in this study or who have not as yet recovered from the toxic effects of such therapy * Patients who underwent surgery within 4 weeks of entering the study or patients who have not as yet recovered from the side-effects of such treatment * Patients with a history of psychiatric illness or condition which could interfere with their ability to understand the requirements of the study (this includes alcoholism/drug addiction) * Patients unwilling or unable to comply with the protocol * Unable to give informed consent * Enrollment in another trial interfering with the endpoints of this study