Approaches to assessing doses to the embryo and fetus (invited paper)

The availability of the required information input for embryo/fetus dosimetry due to internal emitters is far from satisfactory. A variety of anthropometric standards for reference both for national and international agencies have been proposed for assessing healthy in utero growth of the embryo/fetus. Numerous attempts have been made to define computational models of the pregnant woman, embryo/fetus and the placenta for obtaining S factors and SEE values. Human biokinetic information that enables estimation of the time integral of radioactivity in the embryo/fetus is rather scanty and available only for a few radionuclides. However, a number of organisations are working intensively to arrive at doses or dose coefficients due to internal emitters for formulating recommendations that could be adopted by national and international regulatory agencies. The present paper is an attempt to review the different approaches used in embryo/fetus dosimetry. The review includes a comparison of different national standards of anthropometric measurements of the embryo/fetus as a function of gestational period; the modelling of pregnant women, embryo/fetus and the placenta; use of this information for computation of photon-specific absorbed fractions, S factors and SEE values; a brief description of processes involved at placental level that allow or disallow placental transfer of specific materials such as nutrients, micronutrients, trace elements and their subsequent transfer to the embryo/fetus; biokinetic information on either stable or radionuclides and its applicability for computation of radioactivity in the embryo/fetus as well as in its associated component structures or for estimating fetal to maternal activity concentration ratios. A case of an inadvertent administration of therapeutic radioiodine to a pregnant woman is also presented. (author)