We have developed a methodology for calibrating 68Ge radioactivity content in a commercially-available calibration source for dose calibrators in a way that is traceable to the national standard. Additionally, the source was cross-calibrated for equivalent 18F content by direct comparison with the national standard for 18F in the same geometry. Materials and Methods: Sources containing standardized 68GeCl4 or 18F-FDG solutions were prepared at NIST with blanks used in the construction of the RadQual Benchmark epoxy-based 68Ge mock syringe calibration source. These sources and several NIST-constructed epoxy-based 68Ge mock syringes were then used as artifact standards to determine calibration factors for NIST-maintained dose calibrators and secondary standard ionization chambers to enable calibration of the actual Benchmark sources. A direct comparison between the solution-based 68Ge sources and the 18F-FDG sources allowed for an empirical determination of the relative response for these radionuclides in several commercial dose calibrators. Potential measurement effects due to differences between the solution composition and epoxy, as well as theoretical 68Ge/18F response ratios, were studied by Monte Carlo simulation. Results: The calibration factors developed in this study enable NIST to calibrate the RadQual Benchmark source with a relative standard uncertainty of 0.47 %. The direct comparisons of the 68Ge and 18F standards in the various ionizations chambers allowed the activity to be expressed in terms of equivalent 18F activity with a relative standard uncertainty of about 0.5%. Conclusion: The ability for NIST to calibrate the RadQual Benchmark 68Ge source enables direct traceability to the national 68Ge standard to be established for the first time. Through a direct comparison with the NIST 18F standard, the determination of the relative response ratios in dose calibrators enables the equivalent 18F activity to be determined in a way that is also traceable to the national 18F activity standard.
Citation: Journal of Nuclear Medicine
Pub Type: Journals
calibration, germaniun-68, fluorine-18, PET, Standards