Treatment Arm A: Subject will undergo 2 phases of study. In the first phase, termed "dosimetric dose", subjects will receive tositumomab (450 mg) followed by tositumomab (35 mg) that has been trace labeled with 5mCi) Iodine-131 tositumomab. Whole body gamma camera scans will be obtained on day 0, day 2, 3, or 4, and day 6 or 7 following the dosimetric dose. Using the dosimetric data from three imaging time points, a subject-specific dose of iodine I 131 tositumomab to deliver the desired total body dose of radiotherapy will be calculated. In the second phase of the study, termed "therapeutic dose", subjects will receive unlabeled tositumomab (450mg) followed by iodine tositumomab (35mg) labeled with the subject-specific dose of iodine I-131 to deliver a whole body dose of 75 cGy to subjects. Subjects with platelet counts of 100,001 - 149,999 cells/mm3, will receive 65 cGy and subjects who are obese will be doses based on 137% of their lean body mass. Subjects will be treated with either saturated solution potassium iodide (SSKI), Lugol's solution, or potassium iodide tablets starting at least 24 hours prior to the first infusion of the Iodine-131 tositumomab (i.e., the dosimetric dose) and continuing for 14 days following the last infusion of radiolabeled tositumomab (i.e., the therapeutic dose).