Additional Radiation Absorbed Dose Estimates for Zevalin™ Radioimmunotherapy

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Abstract
Zevalin (ibritumomab tiuxetan) radioimmunotherapy is a novel treatment for non-Hodgkin's lymphoma (NHL). The Zevalin regimen includes 5 mCi 111In-labeled Zevalin on Day 1, followed by serial anterior and posterior planar gamma images for imaging or dosimetry. On Day 8, patients receive 0.4 mCi/kg 90Y Zevalin for radioimmunotherapy. Both Zevalin doses are preceded by 250 mg/m2 rituximab to clear peripheral B cells and improve biodistribution of the radiolabeled antibody. In a 143-patient, Phase III, randomized study, the Zevalin regimen produced a significantly higher overall response rate than rituximab for relapsed or refractory, low-grade, follicular, or transformed NHL (80% versus 56%, p = 0.02). Fifteen patients from the Zevalin arm of this study were randomly selected for additional radiation dosimetry. 90Y residence times were calculated from 111In image analysis data. MIRDOSE3.1 radiation absorbed dose estimates to normal tissues were highest for spleen, testes, and liver, with considerably lower doses reaching heart, lung, intestines, red marrow, and kidneys. Radiation absorbed doses to organs and marrow were within a safe range following administration of 0.4 mCi/kg 90Y Zevalin.