Iodine I 131 Tositumomab or Yttrium Y 90 Ibritumomab Tiuxetan in Treating Patients With Non-Hodgkin's Lymphoma

OBJECTIVES: Primary * Determine the relationship between estimated absorbed dose and tumor response using different dosimetric methodologies in patients with non-Hodgkin's lymphoma treated with iodine I 131 tositumomab or yttrium Y 90 ibritumomab tiuxetan. * Determine the relationship between estimated absorbed dose and normal organ toxicity using different dosimetric methodologies in these patients. Secondary * Assess the difference in the dose-response relationship between dosimetric methodologies in these patients. * Assess the influence of prior therapy on the dose-response relationship for hematologic toxicity in these patients. OUTLINE: Patients are stratified according to planned radioimmunotherapy treatment (iodine I 131 tositumomab vs yttrium Y 90 ibritumomab tiuxetan). * Stratum 1: Patients receive dosimetric rituximab IV followed by indium In 111 (\^111In) ibritumomab tiuxetan IV over 10 minutes on day 1. Patients undergo positron emission tomography (PET)/CT scans and single-photon emission computed tomography (SPECT)/CT scans between 2-24, 48-72, and 90-120 hours after \^111In ibritumomab tiuxetan administration. Patients who have acceptable biodistribution receive therapeutic rituximab IV followed by yttrium Y 90 ibritumomab tiuxetan IV over 10 minutes between days 7-9. * Stratum 2: Patients receive dosimetric tositumomab IV over 60 minutes followed by iodine I 131 (\^131I) tositumomab IV over 20 minutes on day 0. Patients undergo PET/CT scans and SPECT/CT scans on days 0; 2, 3 or 4; and 6 or 7. Patients who have acceptable biodistribution receive therapeutic tositumomab IV over 60 minutes followed by \^131I tositumomab IV over 20 minutes on approximately day 7. In both strata, blood is collected at baseline to measure FLT-3 levels. All patients also undergo a baseline PET/CT scan. After completion of study treatment, patients are followed every 3 months for 1 year and then every 6 months thereafter. PROJECTED ACCRUAL: A total of 88 patients will be accrued for this study.