SIR-Spheres Activity Measurements Reveal Systematic Miscalibration

The purpose of this work was to perform an independent and National Institute of Standards and Technology-traceable activity measurement of ⁹⁰Y SIR-Spheres (Sirtex). γ-spectroscopic measurements of the ⁹⁰Y internal pair production decay mode were made using a high-purity germanium detector. Methods: Measured annihilation radiation detection rates were corrected for radioactive decay during acquisition, dead time, source attenuation, and source geometry effects. Detection efficiency was determined by 2 independent and National Institute of Standards and Technology-traceable methods. Results: Measured SIR-Spheres vials (n = 5) contained more activity than specified by the manufacturer calibration; on average, the ratio of measured activity to calibrated was 1.233 ± 0.030. Activity measurements made using 2 distinct efficiency calibration methods agreed within 1%. Conclusion: The primary SIR-Spheres activity calibration appears to be a significant underestimate of true activity.