Radioassay of yttrium-90 radiation using the radionuclide dose calibrator

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 preparations.

A solution of 90YCl3 containing 2.81 kBq/ml (by supplier's calibration) was prepared by serial dilution In 0.05 M HCl. Ten 100-microliters aliquots of this solution were counted in a Packard liquid scintillation analyzer; the mean radioactivity in becquerels was 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 90Y sources at varying solution volumes in plastic and glass containers.

Calibrator readings of 90Y sources in glass and plastic vials and plastic syringes were accurate at either dial setting 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 90YCl3 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 sample volume of 580 microliters.

This work demonstrates the validity of using the 48 x 10 dial-factor combination on the standard radionuclide dose calibrator for calibration of 90Y radiopharmaceuticals.