Imaging and Dosing in Radioimmunotherapy with Yttrium 90 Ibritumomab Tiuxetan (Zevalin)

Yttrium 90 ibritumomab tiuxetan consists of the murine monoclonal antibody ibritumomab securely bound to the second-generation chelator tiuxetan, which attaches the high-energy pure β emitter ⁹⁰Y, for therapy, or the γ emitter indium 111, for imaging. The biodistribution of the therapeutic dose of ⁹⁰Y ibritumomab tiuxetan can be predicted by using an imaging dose of the antibody labeled with ¹¹¹In. Calculation of the therapeutic dose is simple and is based on patient weight and baseline platelet count: for patients with a platelet count of 150 × 10⁹/L or greater the dose of is 0.4 mCi/kg; for patients with mild thrombocytopenia (platelet count 100 × 10⁹/L to 149 × 10⁹/L) the dose is reduced to 0.3 mCi/kg; and the total dose should not exceed 32 mCi. Patients with platelet counts of less than 100 × 10⁹/L should not be treated with ⁹⁰Y ibritumomab tiuxetan. Imaging with ¹¹¹In ibritumomab tiuxetan is performed only to assess biodistribution of the radioimmunoconjugate. Uptake in sites of pathologic adenopathy, as well as other areas of lymphomatous involvement, is frequently seen on the images, but visualization of tumor uptake is not required to proceed with the therapeutic dose of ⁹⁰Y ibritumomab tiuxetan.