The placental transfer of cerium: Experimental studies and estimates of doses to the human fetus from ¹⁴¹Ce and ¹⁴⁴Ce

Purpose: To measure the transfer of cerium from mother to fetus in experimental animals and estimate doses to the human fetus following intakes of radioisotopes of Ce. Materials and methods: Cerium- 141 in chloride solution was administered intravenously to rats at different stages of pregnancy (days 9.5, 12.5 or 18.5), and retention in the embryo/fetus and associated tissues was measured 3 days later in each case. Retention in rat fetal tissues on day 21.5 (shortly before birth) was also measured after administration of ¹⁴¹Ce chloride 1 month prior to conception or ¹⁴¹Ce citrate on day 18.5. Cerium-141 chloride was administered to guinea pigs on day 50 for measurements of fetal retention on day 57 (shortly before birth). Results: Retention of ¹⁴¹Ce in the rat embryo/fetus, measured at 3 days after administration to the mother, increased from about 0.00002% of injected activity per embryo/fetus on day 12.5 to about 0.014% on day 21.5 of gestation. However, the relative concentrations of ¹⁴¹Ce in the embryo/fetus and mother (C_F:C_M ratio) were between 0.005 and 0.01 in each case. After ¹⁴¹Ce administration prior to conception, retention by the rat fetus on day 21.5 was substantially lower than after short-term administration. Comparison of retention of ¹⁴¹Ce on day 21.5 after administration on day 18.5 as either chloride or citrate showed similar levels in maternal tissues but greater transfer to the fetus (C_F:C_M ratio of 0.03). Retention in the guinea pig fetus in late gestation at 7 days after administration of ¹⁴¹Ce chloride was about 0.05% injected activity per fetus, corresponding to a C_F:C_M ratio of about 0.02. Conclusion: These results and other published animal data have been used to specify C_F:C_M ratios for use in the calculation of doses to the human fetus. The values used were 0.05 for intakes during pregnancy and 0.01 for intakes prior to conception. Doses to the offspring after maternal ingestion of ¹⁴¹Ce or ¹⁴⁴Ce are largely due to irradiation from activity in the maternal colon and are insensitive to C_F:C_M. After inhalation, however, absorption of Ce to blood is much greater and doses to the offspring are dominated by the contribution from activity in the fetus, and therefore dependent on the C_F:C_M ratio used.