Inclusion Criteria:

• Must have confirmed CD20+ follicle center lymphoma that meets one of the following:

  1. Histologically confirmed recurrent Revised European American Lymphoma (REAL) Classification CD20+ follicle center lymphoma, follicular grades I and II

  2. Histologically confirmed WHO classification CD20+ follicular lymphoma grades 1, 2, or 3a.

     For either classification, the diffuse component of large cleaved cells (if present) cannot be greater than 50% of cellularity. Patients do not have to express t(14;18) to be eligible.

• Any number of prior regimens (including autologous hematopoietic cell transplantation [HCT]); the most recent prior regimen must have occurred more than 28 days before study entry

• Must demonstrate chemosensitive or radiosensitive disease to most recent prior regimen and meet one of the following criteria:

  1. Patients in second or subsequent complete remission (CR)
  2. Patients in first or subsequent partial remission (PR)
  3. Patients experiencing a relapse that demonstrates a response, as defined as largest nodal mass less than or equal to 3 cm or greater than or equal to 50% reduction in estimated lymph node volume measured as a product of bi-dimensional measurements (see protocol for detailed definition).
  4. Patients with stable follicular lymphoma are eligible if all lymph node masses are less than or equal to 3 cm and are smaller or unchanged in size to the most recent salvage regimen.

• Patients with human leukocyte antigen (HLA)-matched donors that meet the following criteria:

  1. 6/6 HLA-matched related donor. HLA typing must be performed by DNA methods for HLA-A and B at intermediate (or higher) resolution, and DRB1 at high resolution. The donor must be willing to donate peripheral blood stem cells and meet institutional criteria for stem cell donation. The donor must be medically eligible to donate stem cells according to individual transplant center criteria; or,
  2. 8/8 HLA-matched unrelated donor. HLA typing must be performed by DNA methods for HLA-A, B, C, and DRB1 at high resolution. The donor must be willing to donate peripheral blood stem cells and meet National Marrow Donor Program (NMDP) criteria for stem cell donation. The donor must be medically eligible to donate stem cells according to NMDP criteria.

• Patients with adequate organ function, as measured by the following:

  1. Heart: Left ventricular ejection fraction at rest greater than 45%
  2. Lungs: Diffusing capacity of the lung for carbon monoxide (DLCO), forced expiratory volume in one second (FEV1), or forced vital capacity (FVC) greater than 50% of predicted (corrected for hemoglobin). For patients in whom pulse oximetry is performed, baseline O2 saturation greater than 85% (when lung function testing cannot be performed due to age restrictions)
  3. Liver: Bilirubin less than two times the upper limit of normal for age as per local laboratory; alanine aminotransferase (ALT) and aspartate aminotransferase (AST) less than three times the upper limit of normal as per local laboratory
  4. Kidney: Calculated or measured creatinine clearance greater than or equal to 40 mL/min; if creatinine is greater than or equal to 1.5 mg/dL then 24-hour urine for measured creatinine clearance should be performed.

Exclusion Criteria:

• Patients in first CR
• Karnofsky performance score less than 70%
• Patients with follicular lymphoma that demonstrates evidence of histologic transformation. In the presence of B symptoms, rapid growth of a single dominant site, or prolonged (> 2 yrs) interval since last tissue diagnosis, investigators are encouraged to consider re-biopsy of nodes prior to enrollment.
• Uncontrolled hypertension
• Uncontrolled bacterial, viral, or fungal infection (i.e., currently taking medication and progression of clinical symptoms)
• Prior cancer, other than resected basal cell carcinoma or treated cervical carcinoma in situ. Cancer treated with curative intent less than 5 years will not be allowed unless approved by the medical monitor or protocol chair. Cancer treated with curative intent greater than 5 years will be allowed.
• Pregnant or breastfeeding
• Seropositive for human immunodeficiency virus (HIV)
• Fertile men or women unwilling to use contraception from the time of initiation of conditioning until 6 months post-transplant
• Prior allogeneic HSCT
• Known anaphylactic reaction to rituximab
• Seropositive for any of the following: HIV ab, hepatitis B sAg or polymerase chain reaction (PCR)+, or hepatitis C ab or PCR+