We have developed a methodology for calibrating 68 Ge radioactivity content in a commercially available calibration source for activity calibrators in a way that is traceable to the national standard. Additionally, the source was cross-calibrated for equivalent 18 F content by direct comparison with the national standard for 18 F in the same geometry. Methods: Sources containing standardized 68 GeCl 4 or 18 F-FDG solutions were prepared at the National Institute of Standards and Technology (NIST) with mock syringe blanks used in the construction of a commercially available epoxy-based 68 Ge calibration source. These sources and several NIST-constructed epoxy-based 68 Ge mock syringes were then used as artifact standards to determine calibration factors for NIST-maintained activity calibrators and secondary standard ionization chambers to enable calibration of the actual commercial sources. A direct comparison between the solution-based 68 Ge sources and the 18 F-FDG sources allowed for an empiric determination of the relative response for these radionuclides in several commercial activity calibrators. Potential measurement effects due to differences between the solution composition and the epoxy and theoretic 68 Ge-to-18 F response ratios were studied by Monte Carlo simulation. Results: The calibration factors developed in this study enabled NIST to calibrate epoxy-based mock syringe sources with a relative combined standard uncertainty of 0.52%. The direct comparisons of the 68 Ge and 18 F standards in the various ionization chambers allowed the activity to be expressed in terms of equivalent 18 F activity with a relative combined standard uncertainty of about 0.9%. Conclusion: The ability for NIST to calibrate these epoxybased mock syringes enabled, for the first time to our knowledge, the direct traceability to the national 68 Ge standard to be established for this type of source. Through a direct comparison with the NIST 18 F standard, the determination of the relative response ratios in activity calibrators enabled the equivalent 18 F activity to be determined in a way that was also traceable to the national 18 F activity standard. The ability to obtain quantitative data from PET studies has led to an increase in its use in drug discovery. The comparability of results of biodistribution studies across a population of patients in multiple clinical sites using imaging data acquired with different scanners and analyzed with different algorithms requires the data to be linked to a common standard to reliably draw conclusions about differences in patient response. Moreover, the ability to discern small changes in tumor metabolism during a course of treatment can be realized only if there is a way to ensure that the calibrations of all the associated measurement instrumentation (e.g., activity calibrators, commonly known as dose calibrators, and PET scanners) are constant over time.The National Institute of Standards and Technology (NIST) is the national metrology institute of the United States and is responsib...

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After discovering a discrepancy in the transfer standard currently being disseminated by the National Institute of Standards and Technology (NIST), we have performed a new primary standardization of the alpha-emitter 223Ra using Live-timed Anticoincidence Counting (LTAC) and the Triple-to-Double Coincidence Ratio Method (TDCR). Additional confirmatory measurements were made with the CIEMAT-NIST efficiency tracing method (CNET) of liquid scintillation counting, integral γ-ray counting using a NaI(Tl) well counter, and several High Purity Germanium (HPGe) detectors in an attempt to understand the origin of the discrepancy and to provide a correction. The results indicate that a −9.5 % difference exists between activity values obtained using the former transfer standard relative to the new primary standardization. During one of the experiments, a 2 % difference in activity was observed between dilutions of the 223Ra master solution prepared using the composition used in the original standardization and those prepared using 1 mol·L−1 HCl. This effect appeared to be dependent on the number of dilutions or the total dilution factor to the master solution, but the magnitude was not reproducible. A new calibration factor (“K-value”) has been determined for the NIST Secondary Standard Ionization Chamber (IC “A”), thereby correcting the discrepancy between the primary and secondary standards.

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