What question did this study set out to answer?

The aim is to assess various analytical methods for the quantification of actinium-225 and its impurities.

January 20, 2026Open Access

Quality control of actinium-225 and 225Ac-radiopharmaceuticals: Francium-221 to be or not to be?

MTMiguel Toro-GonzálezJapan Radioisotope Association LWLuke WheelessJapan Radioisotope Association VTVijai Kumar Reddy Tangadanchu

Key Points

The aim is to assess various analytical methods for the quantification of actinium-225 and its impurities.
Tested five instruments for linearity, sensitivity, accuracy, and specificity.
Examined francium-221 and bismuth-213 equilibria for actinium-225 quantification.
Analyzed TLC RCY% using HPGe and gas-filled proportional counter.
Francium-221 equilibrium was accepted for TLC RCY% using particular instruments.
Gamma well counting, LSC, and plastic silicon detector needed bismuth-213 equilibrium for accuracy.
Low levels of actinium-225 impurity were reported accurately, except by the silicon detector.

Abstract

Five instruments have been tested for their linearity, sensitivity, accuracy and specificity to actinium-225 quantification using francium-221 and bismuth-213 equilibria. Francium-221 equilibrium was deemed acceptable for TLC RCY% using HPGe and gas-filled proportional counter. Gamma well counting, LSC, and plastic silicon detector required bismuth-213 equilibrium for accurate RCY%. Low content free actinium-225 impurity was accurately reported for all methods except for the silicon detector. Overall, this investigation sheds light on the appropriate analytical methods for 225Ac-radiopharmaceutical QC considering secular equilibrium.

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