A methodological study on measuring the activity of Ac-225 in targeted therapeutic radiopharmaceuticals using CdZnTe

Ac-225 is a promising radionuclide for clinical α-particle targeted cancer therapy. Accurate measurement of its activity is prerequisite for clinical applications. To explore viable methods for Ac-225 activity measurement in clinical settings, this study used a CdZnTe detector to measure the γ-ray spectra of Th-229 solutions, investigated factors influencing measurement results, and developed efficiency calibration methods. Results indicate that: 1. Only the 440.46 keV γ-ray emitted by daughter nuclide Bi-213 and the 218.19 keV γ-ray emitted by Fr-221 are suitable for analyzing Ac-225 activity; 2. Significant discrepancies (up to 36.8%) exist in γ-ray emission intensities of Ac-225 and its daughters across different databases, necessitating emission intensity corrections in calculations; 3. When calibrating CdZnTe detection efficiency using the Eu-152 standard source method and Monte Carlo simulation, deviations from reference values were 2.0% and -4.6%, respectively, confirming the reliability of both methods; 4. The impact of cascade coincidence-summing effects on CdZnTe measurements is negligible, whereas it can reach 32.4% for HPGe detectors. These findings provide critical references for establishing standardized techniques to measure Ac-225 activity in radiopharmaceuticals using CdZnTe detectors.