Traceable calibration with ¹⁷⁷Lu and comparison of activity meters at hospitals in Norway and Sweden

Introduction: The activity meter is used to determine the activity of delivered radiopharmaceuticals, administered activity to patients and reference activity when gamma-cameras are calibrated prior to imaged-based dosimetry. The aim is to describe a procedure for cross-calibration of activity meters at different clinical sites, and report on ¹⁷⁷Lu activity results when using factory-set calibration factors compared to when calibration is performed with traceability to a primary standard. Methods: Thirty activity meters placed at seven hospitals in Norway and Sweden from four manufacturers: Capintec, Commecer, NuviaTech and Veenstra were included. A stock solution with ¹⁷⁷Lu was prepared at the local sites and measured in each activity meter with factory settings. The solution was shipped to the reference site at Lund University for measurements in a secondary standard activity meter. Deviations between local and reference activity measurements were determined for three geometries: 25-mL vial, 10-mL syringe and 1-mL syringe. Results: The median of the deviations was 6.4 % for the 25 mL vial, 5.9 % for the 10 mL syringe and 6.8 % for the 1 mL syringe. The median of the deviations for the 25 mL vial, was 1.5 % for activity meters from Capintec, 7.0 % for Comecer, 11.0 % for NuviaTech and 2.4 % for Veenstra. The majority of the deviations were positive and the maximum deviation was 14.5 %. Conclusion: The activity of ¹⁷⁷Lu measured in an activity meter with factory-set dial settings may yield deviations up to 14.5%, compared to activities measured with traceability to a primary standard. This would imply an undertreatment of patients.