Radionuclide calibrator responses for 224Ra in solution and adsorbed on calcium carbonate microparticles
Denis E. Bergeron, Elisa Napoli, Jeffrey T. Cessna, Leticia S. Pibida, Ryan P. Fitzgerald, Gro Hjellum
A suspension of 224Ra adsorbed onto CaCO3 microparticles shows promise for localized internal α‐therapy of intracavitary micro-metastatic diseases, i.e., peritoneal carcinomatosis (PC) originating from ovarian cancer, colorectal cancer, or other malignancies. To facilitate accurate administrations and reliable dose calculations, the National Institute of Standards and Technology (NIST) has developed geometry-specific calibration factors (or dials settings) for some commercially available radionuclide calibrators, i.e., reentrant ionization chambers. Methods: The calibration factors were determined for 224Ra in secular equilibrium with its progeny as 224RaCl2 solutions and 224Ra adsorbed onto CaCO3 microparticles suspended in water. To evaluate geometry effects (container size and sample composition) on ionization chamber response, a variety of sources were prepared from a common master solution and measured. Microparticles were labeled in situ with high efficiency and activity retention (> 99 %). Monte Carlo calculations supported interpretation of the underlying physics. Results: Ampoules and vials give ionization chamber responses consistent to < 1 %. Attenuation by microparticles results in a reduction in response varying from ≈ 1 % to ≈ 2.5 % in the geometries studied. Ionization chamber calibration settings for 224RaCl2 solution and 224Ra adsorbed onto CaCO3 microparticles, in ampoules, vials, and syringes, were determined. The observed effects of geometry and attenuation on ionization chamber responses are interpreted so that predictions can be made when new formulations or containers are encountered.