SIR-Spheres Activity Measurements Reveal Systematic Miscalibration

Area of Science:

• Nuclear Medicine
• Radiopharmacology
• Medical Physics

Background:

• Selective Internal Radiation Therapy (SIRT) utilizes Yttrium-90 (90Y) SIR-Spheres for liver cancer treatment.
• Accurate quantification of radioactive activity in these brachytherapy sources is crucial for effective and safe patient dosing.
• Previous calibrations by manufacturers may require independent verification.

Purpose of the Study:

• To conduct an independent and traceable activity measurement of 90Y SIR-Spheres.
• To verify the accuracy of manufacturer-provided activity calibrations for 90Y SIR-Spheres.

Main Methods:

• Utilized gamma-spectroscopic measurements with a high-purity germanium detector to analyze the 90Y internal pair production decay mode.
• Corrected raw detection rates for radioactive decay, dead time, attenuation, and geometric effects.
• Determined detection efficiency using two independent methods traceable to the National Institute of Standards and Technology (NIST).

Main Results:

• Measured activity in five SIR-Spheres vials exceeded the manufacturer's specified calibration values, with an average measured-to-calibrated activity ratio of 1.233 ± 0.030.
• Activity measurements obtained using two distinct efficiency calibration methods demonstrated excellent agreement, with results within 1% of each other.
• The findings indicate a consistent overage of radioactive activity compared to manufacturer calibrations.

Conclusions:

• The primary activity calibration for SIR-Spheres appears to significantly underestimate the true radioactive content.
• Independent verification of brachytherapy source activity is essential for ensuring accurate therapeutic doses.
• Further investigation into the calibration procedures for 90Y SIR-Spheres may be warranted.