223Ra is a bone-seeking alpha emitter, and as such is currently being evaluated as a radiopharmaceutical for the treatment of skeletal metastases. In the clinical setting, dosage measurements are typically achieved with reentrant ionization chambers referred to as dose calibrators . In order to achieve acceptable levels of accuracy with these chambers, appropriate calibration factors, or dial settings , must be employed. Concurrent with the primary standardization of 223Ra, the Radioactivity Group at NIST established that the calibration factors currently in use for clinical trials give average activity readings 5.7% to 8.7% higher than the NIST calibrated activity. The present development of secondary standards determines calibration settings for the most commonly used dose calibrators for the standard 5 mL NIST ampoule geometry and for clinically relevant geometries. Because the characteristics (wall thickness, composition, etc.) of the sample container affect the extent to which source radiation is attenuated, accurate activity measurements must be geometry-specific. Calibration factors were determined for each of the five NIST maintained chambers. The best dial settings for the 5 mL NIST ampoule and the 20 mL dose vial geometries differed by less than 1 % for all but one of the chambers. For the one exception, the difference was 1.14 % and the calibration factors agreed to within the experimental uncertainty (+/- 0.76 %). For the 20 mL dose vial geometry, volume effects from 0.5 mL to 6 mL were insignificant with respect to the experimental uncertainty. 2 mL, 5 mL, and 20 mL syringe geometries were also evaluated, and will be discussed.
Citation: Applied Radiation and Isotopes
Pub Type: Journals
dose calibrator, Radiu223, secondary standard