Biokinetic model for 137Cs in the fetus.

The metabolic data for cesium and the biologically similar element potassium in the pregnant woman were reviewed. Due to the rapid growth of the placenta and fetus, the pregnancy was divided into four gestational periods: five through ten weeks, eleven through twenty weeks, twenty-one through thirty weeks, and thirty-one through forty weeks. A physiologically based biokinetic model for potassium was developed for each of these periods. This model was used as a basis for the cesium model. The models consisted of several organs, all of which involved recycling and for which it was assumed that equilibrium existed between the uterus and maternal plasma and the fetal organs and fetal plasma. The absorbed dose to the fetus and placenta was estimated from 137Cs located in the fetus and placenta using medical internal radiation dosimetry methodology. The dose to the embryo/ fetus was 1.3 x 10(-5) Gy Bq(-1) during the first gestational period and decreased to 8.3 x 10(-6) Gy Bq(-1) for the final period. The dose to the placenta also decreased as the pregnancy progressed. 137Cs located in the maternal organs was not accounted for when determining the fetal dose estimates.