Activity Assays For The Alpha-Emitting Radionuclide Ra-224 Adsorbed Onto Caco3 Microparticles

1390 Objectives: The National Institute of Standards and Technology (NIST) recently developed an activity standard for 224Ra in equilibrium with its progeny. The standard supported the direct calibration of 224RaCl2 solutions in ionization chambers (ICs) and calibration settings for numerous commercial radionuclide calibrators (“dose calibrators”) were reported. One promising therapeutic use of 224Ra delivers the alpha-emitter to cavity regions via direct injection of a suspension of CaCO3 microparticles with surface-adsorbed 224Ra. Photon attenuation by CaCO3 can be expected to affect activity assays of 224Ra adsorbed onto microparticles and in this work we describe measurements of these effects, providing the foundation for accurate clinical administrations and reliable dose calculations. Methods: A solution was calibrated for activity concentration using the IC factors established during the primary standardization. We developed an in situ approach for adsorbing 224Ra onto CaCO3 microparticles so that it was possible to dispense gravimetrically linked aliquants of the calibrated master solution to a series of sources containing 1 mol/L HCl or microparticles. With precise knowledge of the total 224Ra activity dispensed to each source, differences in relative IC responses could be understood in terms of attenuation by microparticles and/or container effects. Results: The in situ labelling efficiency and activity retention were determined to be > 99 %. IC calibration settings for 224RaCl2 solution and 224Ra adsorbed onto CaCO3 microparticles in vials and syringes were determined, enabling activity measurements with typical combined standard uncertainties Conclusions: Radionuclide calibrator settings for 224RaCl2 solutions and 224Ra adsorbed on CaCO3 microparticles were determined for vials and syringes. Photon attenuation by the microparticles significantly (> 2σ) affected activity readings. Using composition-specific calibration settings allows activity assays with