TY - JOUR T1 - SIR-Spheres activity measurements reveal systematic miscalibration JF - Journal of Nuclear Medicine JO - J Nucl Med DO - 10.2967/jnumed.121.262650 SP - jnumed.121.262650 AU - Stephen A Graves AU - Molly Martin AU - Ashok Tiwari AU - Michael J Merrick AU - John J Sunderland Y1 - 2022/01/01 UR - http://jnm.snmjournals.org/content/early/2022/01/06/jnumed.121.262650.abstract N2 - Purpose: Accurate dosimetry-guided radiopharmaceutical therapy fundamentally relies on knowledge of the quantity of radioactivity administered to patients. The purpose of this work was to perform an independent and NIST-traceable activity measurement of 90Y SIR-Spheres®. Methods: Gamma spectroscopic measurements of the 90Y internal pair production decay mode were made using a high-purity germanium detector. Un-modified patient SIR-Spheres® vials were placed within a high-density polyethylene source holder positioned at a distance of 210 cm from the detector, with acquisition durations of 3 – 6 hours. 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 two independent and NIST-traceable methods. Resulting 90Y activity measurements were compared against the manufacturer activity calibration. Results: Measured SIR-Sphere® vials (n = 5) were found to contain 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 two distinct efficiency calibration methods were found to agree within 1%. Uncertainty in individual measurements was dominated by counting statistical uncertainty (~2.5%), uncertainty in the internal pair production branching ratio of 90Y (1.5%), and efficiency calibration (1.2% – 1.9%). Conclusion: The primary SIR-Spheres® activity calibration appears to be a significant underestimate of true activity. This mis-calibration has likely been consistent for as long as the SIR-Sphere® product has been available. This finding should be independently verified, and steps should be taken by the manufacturer to establish an accurate and traceable activity standard. ER -