RT Journal Article
SR Electronic
T1 A primary activity standard for the alpha-emitting radionuclide Ra-224.
JF Journal of Nuclear Medicine
JO J Nucl Med
FD Society of Nuclear Medicine
SP 1191
OP 1191
VO 60
IS supplement 1
A1 Elisa Napoli
A1 Jeffrey Cessna
A1 Ronald Collé
A1 Ryan Fitzgerald
A1 Lynne King
A1 Lizbeth Laureano-Pérez
A1 Leticia Pibida
A1 Brian Zimmerman
A1 Denis Bergeron
YR 2019
UL http://jnm.snmjournals.org/content/60/supplement_1/1191.abstract
AB 1191Objectives: Radium-224 is an alpha-emitting radionuclide that when adsorbed on calcium carbonate microparticles, has been shown to be efficacious in the treatment of metastatic cancers in cavity regions in preclinical models. With a 3.631(2) d half-life and a decay scheme that includes the emission of four energetic alpha particles and two energetic beta particles, radiopharmaceuticals based on 224Ra can deliver a very high dose over a short range, resulting in minimal toxicity to surrounding tissues. To facilitate accurate administrations and reliable dose calculations, the National Institute of Standards and Technology (NIST) has developed a radioactivity standard for 224Ra in equilibrium with its daughters and used it to develop calibrations for instruments commonly encountered in clinical settings. Methods: Solutions of 224RaCl2 in 1 mol/L HCl were received and assayed by multiple methods. Gamma spectroscopy with high-purity germanium (HPGe) detectors placed limits on photon-emitting impurities and confirmed equilibrium with daughters. The primary activity standard was based on measurements by live-timed anticoincidence counting (LTAC). Confirmatory measurements included two primary liquid scintillation counting methods. In addition, samples were measured on multiple ionization chambers (IC) and well-type NaI(Tl) counters (WC). Monte Carlo simulations were used to predict IC and WC responses. Results: The LTAC-based activity standard carries a 0.30 % (k = 1) combined standard uncertainty, with excellent agreement between methods. Simulated IC and WC responses were used to calculate activity concentration values that agreed with the LTAC-based values to within their uncertainties. Calibration factors were also determined for multiple clinical ICs (i.e., “radionuclide calibrators” or “dose calibrators”). Because 224Ra has a complex decay scheme, it takes several days after separation to reach secular equilibrium with its daughters. Using the IC Monte Carlo models and daughter fractions based on the Bateman Equations, daughter ingrowth could be modeled precisely. Fits of the data with the chemical separation time as the only variable reproduced the manufacturer’s stated separation time to within a few hours; leaving the 224Ra half-life as an additional variable reduced small trends in the fit residuals. Conclusions: NIST has developed an activity standard for 224Ra in equilibrium with its daughters. The standard has been used to calibrate clinical instruments. Ongoing work will consider clinically relevant measurement geometries and will include nuclear data (i.e., half-life, gamma-ray emission probabilities) measurements.