Impact of Recent Change in the National Institute of Standards and Technology Standard for 18F on the Relative Response of 68Ge-Based Mock Syringe Dose Calibrator Standards.

Abstract

As a result of a recent change in the National Institute of Standards and Technology (NIST) activity standard for (18)F, we have determined new relative response ratios for a (68)Ge solid epoxy mock syringe source used in activity calibrators as a long-lived substitute for (18)F. 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. Following our previously published methodology, solution-filled mock syringe sources, identical in geometry to the solid (68)Ge epoxy calibration source currently on the market, were prepared using NIST-calibrated solutions of (68)GeCl4 and (18)F-FDG and directly compared in several models of activity calibrators to determine empirically the relative response ratios for these 2 radionuclides. The new relative response ratios measured in this study reflect the change in (18)F activity measurements that arise from the recent -4% change in the NIST activity standard. The results allow the (68)Ge activity of the mock syringe source to be expressed in terms of equivalent (18)F activity, with a relative combined standard uncertainty of about 0.8% for the activity calibrators used in this study. This work revises our previously derived relative response ratios for (18)F and (68)Ge 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 (18)F standard.