Impact Of Recent Change In The National Institute Of Standards And Technology Standard For F-18 On The Relative Response Of Ge-68-Based Mock Syringe Dose Calibrator Standards

As a result of a recent change in the National Institute of Standards and Technology (NIST) activity standard for F-18, we have determined new relative response ratios for a Ge-68 solid epoxy mock syringe source used in activity calibrators as a long-lived substitute for F-18. New standardized solutions of each radionuclide were used to determine the response ratios while maintaining traceability to national standards. This work updates our previously published data from 2010. Methods: Following our previously published methodology, solution-filled mock syringe sources, identical in geometry to the solid Ge-68 epoxy calibration source currently on the market, were prepared using NIST-calibrated solutions of (GeCl4)-Ge-68 and F-18-FDG and directly compared in several models of activity calibrators to determine empirically the relative response ratios for these 2 radionuclides. Results: The new relative response ratios measured in this study reflect the change in F-18 activity measurements that arise from the recent -4% change in the NIST activity standard. The results allow the Ge-68 activity of the mock syringe source to be expressed in terms of equivalent F-18 activity, with a relative combined standard uncertainty of about 0.8% for the activity calibrators used in this study. Conclusion: This work revises our previously derived relative response ratios for F-18 and 68Ge by -3.7%, allowing users of the commercial mock syringe surrogate source to calibrate their activity calibrators in a way that is consistent with the recent change in the NIST F-18 standard.