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Radioassay of Yttrium-90 Radiation Using the Radionuclide Dose Calibrator

Molecular Cancer Institute, Department of Internal Medicine, University of California, Davis, Sacramento, California

Correspondence: For correspondence or reprints contact: Qansy Salako, PhD, Molecular Cancer Institute, 1508, Alhmabra Blvd., Sacramento, CA 95816.

ABSTRACT

Yttrium-90 is used in radioimmunotherapy because of its favorable physical half-life and energetic pure beta emissions. However, it is often necessary to standardize ⁹⁰Y sources to establish a dose calibrator dial setting for accurate calibration of clinical doses of ⁹⁰Y reparations. Methods: A solution of ⁹⁰YCI₃ containing 2.81 kBq/ml (by supplier's calibration) was prepared by serial dilution in 0.05 M HCI. Ten 100-µl aliquots of this solution were counted in a Packard liquid scinlillation analyzer; the mean radioactivity in becquerals cwas determined and used to evaluate dial settings 48 x 10,775 x 70 and 775 x 100 on a radionuclide dose calibrator for ⁹⁰Y measurements. The dose calibrator response was also studied on sources at varying solution volumes in plastic and glass containers. Results: Calibrator readings of ⁹⁰Y sources in glass and plastic vials and plastic syringes were accurate at aither dial sethng 48 x 10 (commonly used by many ⁹⁰Y laboratories) or 775 x 70. Measurements of 1.15 and 3.03 GBq (31 and 82 mCi, respectively) calibrated ⁹⁰Y sources in either vial were –3.0 and +4.3%, respectively, at dial-setting 775 x 70 and –4.0 and +9.0% at 48 x 10. Yttrium-90 sources in plastic syringes gave higher readings than those in glass vials, therefore, requiring a container correction factor for accurate dose assay. Measurements of ⁹⁰YCI₃ shipments from four suppliers over a 3-yr period demonstrated concurring calibration measurements at both 775 x 70 and 48 x 10 settings for shipments from all suppliers. The dose calibrator response to ⁹⁰Y radiation was linear within a 1–333 kBq range in a constant sampiS volume of 580 µl. Conclusion: This work demonstrates the validity of using the 48 x 10 dial-factor combinationon the standard radionuclide dose calibrator for calibration of ⁹⁰Y radiopharmaceuticals.

Key Words: yttrium-90 • dose calibrator • assay • liquid scintillation • linearity test