Estimation of absorbed doses in computational mouse and human phantoms receiving229Th-derived free225Ac.

Objective. Free 225Ac can result in off-target radiation exposure if present as a result of incomplete chelation or radiolytic degradation of 225Ac radiopharmaceuticals. This study assessed absorbed doses of 229Th-derived free 225Ac to major normal tissues and organs in mouse and human models.Approach. Biodistribution data were obtainedex vivofrom SK-MEL-2 xenograft mice administered with [225Ac]AcCl3derived from a229Th/225Ac generator. Uptake fractions of225Ac in selected tissues and organs were measured at 0.5, 2, 6, and 24 h postinjection (p.i.) via its gamma-emitting daughter,221Fr, with a sample size ofn = 4-5 per p.i. time point. Time-integrated activity coefficients (TIACs) were calculated by fitting the mouse uptake profiles and extrapolated to human models using relative mass scaling. Absorbed dose coefficients (ADCs) were then calculated according to the Medical Internal Radiation Dose formalism using S values of a mouse whole-body (MOBY) phantom and anthropomorphic adult male and female phantoms implemented in the OLINDA dosimetry software. Redistribution of221Fr and213Bi from the liver to kidneys was simulated by adjusting progeny-specific TIAC ratios in the mouse model.Main results. The liver received the highest absorbed dose, with ADCs of 2860 mGy kBq-1in the MOBY phantom and 886-1080 mGy MBq-1in the anthropomorphic phantoms. The skeletal and splenic ADCs were 84.3 and 20.3 mGy kBq-1, respectively, in the MOBY phantom. ADCs in the other tissues and organs were low to moderate, with the brain receiving the least. Under simulated redistribution scenarios,221Fr showed larger changes in mouse ADCs than213Bi, increasing the renal ADCs by 7.5-30.5% and decreasing the hepatic ADCs by 8-31.8% for scale factors of 0.1-0.4.Significance. The liver, bone, and spleen were suggested as the primary dosimetric targets of229Th-derived free225Ac. The ADCs of free225Ac calculated in this work may serve as a preliminary reference for quality control of225Ac radiopharmaceuticals.