A new experimental determination of the dose calibrator setting for 188Re.

UNLABELLED Accurate activity measurements of radionuclides using commercial dose calibrators requires that the correct dial setting (or calibration factor) be applied. The dose calibrator setting for the medical radionuclide 188Re (as 188ReO4-) has been determined experimentally using solution sources prepared and calibrated at the National Institute of Standards and Technology (NIST). METHODS The specific activity of two sources (in units of MBq/g) in the standard 5-mL NIST ampoule and in a 5-mL SoloPak dose vial were calibrated using 4pibeta liquid scintillation counting with 3H-standard efficiency tracing and gamma-ray/bremmstrahlung counting in the NIST "4pi" gamma ionization chamber on gravimetrically related sources. RESULTS The newly determined settings for the NIST Capintec CRC-12 dose calibrator are (631+/-4) x 10 and (621+/-3) x 10 for the respective ampoule and dose vial geometries with an expanded (at a presumed 95% confidence level) uncertainty of 0.4%-0.5% in the activity determination. The setting for the dose vial geometry was independently confirmed using a Capintec CRC-15R at Cedars-Sinai Medical Center using sources calibrated against a NIST standard. CONCLUSION These new settings result in activity readings 28%-30% lower than those obtained using the previously recommended setting of 496 x 10. This discrepancy most likely results from underestimating the total radiation yield from 188Re decay when calculating the dose calibrator response. This study emphasizes the need for experimental determinations of dose calibrator settings in the geometry in which the measurements will be performed.