Sexual dimorphism in ¹³⁷Cs accumulation after chronic low dose exposure in mice

The presence of Cesium-137 (¹³⁷Cs) in the environment after nuclear accidents at Chernobyl and Fukushima Daiichi raises many health issues for the surrounding populations chronically exposed through the food chain. Unlike previous in vivo studies that focused solely on male exposures, this experimental research aims to assess the biodistribution and dosimetry of low-dose ¹³⁷Cs internal exposure in both male and female C57BL/6 mice. This study uses a previously established model for chronic ingestion of ¹³⁷Cs, simulating various exposure scenarios. Male and female C57BL/6 mice were exposed to concentrations resembling those ingested daily by residents in contaminated areas (20 kBq/L), as well as 5- and 10-times higher concentrations, for either 6 or 24 weeks. These exposure periods were chosen to assess both short-term and long-term effects of chronic ¹³⁷Cs exposure, allowing us to observe differences in ¹³⁷Cs accumulation and elimination in mice. Throughout this period, the animals were closely monitored to determine and quantify the ¹³⁷Cs content and to calculate absorbed doses. After 6 or 24 weeks exposure to chronic ¹³⁷Cs in drinking water at 500 kBq/L, ¹³⁷Cs concentration varied according to the organs and the sex of the animals. Males showed a higher body burden of ¹³⁷Cs compared to females, with significant differences observed as soon as day 11. As well, the organs showing the highest ¹³⁷Cs concentrations were skeletal muscle in males and in females, with significant differences between males and females. Regarding excretion, it appears that the elimination of ¹³⁷Cs through feces was similar in males and females. By contrast, female mice showed a higher rate of ¹³⁷Cs urine excretion than males, thus explaining the lower body burden in females. The resulting absorbed doses, calculated using dose conversion factors provided by ICRP publication 108, showed that the absorbed dose is 1.85 times less in female mice compared to male mice. 36.1 mGy in females and 66.9 mGy in males after 6 weeks exposure. 182.0 mGy in females and 310.0 mGy in males after 24 weeks exposure. This study demonstrates for the first time, sexual dimorphism in ¹³⁷Cs biokinetics between males and females. These findings could refine biokinetic models of cesium and absorbed dose estimations in case of internal contamination, especially in post-accidental situations.