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

Accurate activity measurements of radionuclides using commercial dose calib rators requires that the correct dial setting (or calibration factor) be ap plied. The dose calibrator setting for the medical radionuclide Re-188 (as (ReO4-)-Re-188) has been determined experimentally using solution sources p repared and calibrated at the National Institute of Standards and Technolog y (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 cal ibrated using 4 pi beta liquid scintillation counting with H-3-standard eff iciency tracing and gamma-ray/bremmstrahlung counting in the NIST "4 pi" ga mma ionization chamber on gravimetrically related sources. Results: The new ly determined settings for the NIST Capintec CRC-12 dose calibrator are (63 1 +/- 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) uncertain ty of 0.4%-0.5% in the activity determination. The setting for the dose via l geometry was independently confirmed using a Capintec CRC-15R at Cedars-S inai Medical Center using sources calibrated against a NIST standard. Concl usion: These new settings result in activity readings 28%-30% lower than th ose obtained using the previously recommended setting of 496 x 10. This dis crepancy most likely results from underestimating the total radiation yield from Re-188 decay when calculating the dose calibrator response. This stud y emphasizes the need for experimental determinations of dose calibrator se ttings in the geometry in which the measurements will be performed.