Updated NIST standards for dose calibrator measurements of imaging and therapeutic nuclides

Objectives To preserve activity standards for short-lived radionuclides, the National Institute of Standards and Technology (NIST) establishes calibration factors for reentrant ionization chambers (ICs). In nuclear medicine clinics, similar ICs are referred to as “dose calibrators”, and nuclide-specific calibration factors are referred to as “dial settings”. Over the past few years, NIST has determined dial settings for new instruments and geometries for nuclides such as Lu-177 and In-111 and updated settings as primary standards for some nuclides have been revisited.

Methods Dial setting determinations were performed on the NIST-maintained set of dose calibrators; this set comprises eight instruments from three manufacturers. In all cases, calibration factors were determined for the NIST standard 5 mL ampoule. For Lu-177 and Ra-223, we also considered clinical geometries. Standard activities were determined by reference to earlier NIST standards or in some cases by direct assay by primary methods.

Results Activities determined with the new calibration factors for F-18, based on the new NIST primary standard (which is in good accord with standards from other National Metrology Institutes), differ from the earlier NIST standard by +4.0 %. New calibration factors for In-111 give activities approximately 3 % higher than the manufacturer-recommended settings. For I-123, we found an 11.3 % spread in activities (from -3.3 % to +8.0 %, relative to the NIST standard activity) using different manufacturers’ recommended settings.

Conclusions NIST-determined dose calibrator dial settings provide an important point of comparison for clinical measurements. Some of the measurement biases identified here exceed 10 %, making intercomparability of quantitative imaging data impossible and dose calculation inaccurate. NIST strives to disseminate accurate standards to increase confidence in clinical measurements, reduce measurement uncertainties, and facilitate truly comparable values in quantitative molecular imaging.