I-131-tositumomab also requires a trace-labeled infusion prior to the therapeutic dose. Unlike the Y-90-ibritumomab tiuxetan, I-131-tositumomab trace infusion is used to calculate the therapeutic dose. For this purpose, 450 mg of unlabeled tosi-tumomab is infused over 60 minutes. This is then immediately followed by an intravenous infusion of 35 mg of tositumomab radioiodinated with 5 mCi of I-131 given over 20 minutes. Patients then undergo total body gamma counts within one hour of the dosimetry infusion and then two to four days and six to seven days after infusion of the dosimetric dose. If the biodistribution is altered, such as the absence of predominance of the blood pool early postinfusion or localization to nontarget sites, the therapeutic infusion should not be administered. Importantly, the determination of the total body residence time is derived from the total body gamma counts. The appropriate mCi dose of I-131 is determined to deliver the desired TBD of either 75 cGy or 65 cGy, depending on the patient's platelet count as described in Equation 1. For the therapeutic dose, again 450 mCi of unlabeled tositumomab is infused over 60 minutes. This is then immediately followed by 35 mg of tositumomab radioiodinated with the desired dose of I-131 to deliver the appropriate TBD. Following infusion, one must note that appropriate restrictions on patient contact and discharge from the hospital must follow local, federal, and state guidelines. Finally, all infusions should be premedicated with acetaminophen and diphenhydramine and patients should be prophylaxed with a thyroid protective agent such as super saturated potassium iodide (SSKI) or Lugol's solution for at least 24 hours prior to administration of the first dose of I-131-tositumomab, and continued until two weeks after the last administration of I-131-tositumomab.

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