Dosimetry Considerations Y90Ibritumomab Tiuxetan

The use of Y-90-ibritumomab tiuxetan does not require formal dosimetry calculations when used according to FDA-approved guidelines. This approach uses a mCi/kg therapeutic dose, but does require In-111 imaging to ensure that an altered biodistribution does not occur. To evaluate for such an altered biodistribution, patients are given trace-labeled In-111-ibritumomab tiuxetan following the rituximab cold infusion. They then undergo serial whole-body gamma camera imaging over several following days to ensure that early postinfusion of the majority of the radioimmunoconjugate remains within the blood pool and over time localizes to target tumor sites. If there is no evidence for an altered biodistribution, patients can then be treated with 0.4 mCi/kg of Y-90 up to a maximum of 32 mCi with this approach. Patients with platelet counts between 100,000 and 150,000 should be treated with 0.3 mci/kg of Y-90-ibritumomab.

In contrast, the use of I-131-tositumomab requires whole-body dosimetry. Patients receive a cold dose of 450 mg of tositumomab, followed by a trace-labeled hot dose of approximately 35 mg of tositumomab labeled with 5 mCi of I-131. Patients then undergo serial whole-body gamma counts on days 0, 2, 3, or 4, and days 6 or 7 following trace infusion. From these results, a calculation as to the appropriate mCi dose of I-131 can be achieved to deliver 75 cGy whole-body radiation dose to patients with normal platelet counts and 65 cGy whole-body dose to patients with platelet counts between 100,000 and 150,000.

Calculation of therapeutic dose of I-131-tositumomab is given in Equation 1. [Activity hours are derived from patient mass and reference table. Residence time represents 37% of residual whole body activity as derived from a semilog plot of percent injected whole body activity. Desired total body dose (TBD) is either 75 cGY or 65 cGy depending on platelet count.]

Activity hours (mCi)

Therapeutic dose (mCi) =-

Residence time (hr)

Desired TBD (cGy)

75 cGy

Patients are then infused with the therapeutic dose in the same algorithm consisting of a cold 450 mg unlabeled anti-CD20 antibody dose followed by the labeled 35 mg dose.

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