In nuclear medicine clinics, activity measurements are typically made using commercially available radionuclide calibrators which incorporate a reentrant 4Π ionization chamber and an electrometer.
The current measured by these calibrators is translated into activity units by applying a “dial setting” (calibration factor) specific to the nuclide under investigation. Specific manufacturer-recommended dial settings are often determined theoretically by extrapolating from a response curve based on limited experimental data. In addition, they are often determined for a geometry (5 mL ampoule) not often employed in clinical settings. The Radioactivity Group at NIST maintains a collection of commercial radionuclide calibrators, including models from each of the major U.S. manufacturers. Using these instruments, NIST can link dial settings for a given nuclide in a given geometry to national standards for activity.
Following a recent primary standardization as part of an international Key Comparison, a dial setting determination was performed for 177Lu. With three primary β- branches and a 6.647 d halflife, 177Lu is an appealing radiotheraueutic. When complexed to DOTA-TATE, it binds to the somatostatin receptors found on the surface of tumor cells, enabling highly specific targeted radiotherapy. In this case, the manufacturer-recommended dial settings for the standard 5 mL ampoule geometry (where available) were in good accord with those determined by NIST. In 2012, NIST participated in an intercomparison piloted by the National Physical Laboratory (UK); most of the participants in this exercise were clinical sites in the UK. As part of this exercise, NIST determined dial settings specific to the clinically common 10R Schott vial geometry. In this case, it was found that using the ampoule-specific dial setting to measure a 10R Schott vial would not introduce significant error (< 0.73 %) to the measurement.
The quality control procedures on the NIST-maintained radionuclide calibrators are being increasingly automated, and new control chart-based algorithms are being applied.