SIR-Spheres activity measurements reveal systematic miscalibration

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 Y SIR-Spheres®. Gamma spectroscopic measurements of the Y 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 Y activity measurements were compared against the manufacturer activity calibration. Measured SIR-Sphere® vials ( = 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 Y (1.5%), and efficiency calibration (1.2% - 1.9%). 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.