Endocrine Abstracts

Purpose: Based on ICRP recommendations, accidental exposure of embryo/fetus to radiation at any gestational age during [¹³¹I]NaI therapy may result in wide range of genetic damage, cancer, and early death of the embryo/fetus. More than the limitation in clinical study due to ethical aspects, the previous studies performed radiation dosimetry in embryo/fetus using [¹³¹I]NaI biodistribution in animal and Medical Internal Radiation Dose (MIRD) method with their uncertainties. Here we estimate the embryo/fetus radiation absorbed dose from maternal organs and thyroid gland of fetus in primary gestation age by [¹³¹I]NaI using a more accurate Monte Carlo method.

Methods: An ICRP 110 digital female phantom with an embryo/fetus in the first 13 weeks’ gestation was used, considering ¹³¹I energy spectrum for auger, β, γ and x-ray. The maternal uterus, thyroid, and ovary and embryo/fetus thyroid >9 weeks gastation were considered as sources and total embryo/fetus volume was considered as target. The factor [mGy/(MBq.h)], which represents the absorbed doses delivered to target per unit disintegration of radionuclide in source regions was calculated using a Monte Carlo method in 6-13 weeks gestation. This method provides a more realistic model for interaction of decayed particles and photons with tissue. All simulations were performed with GATE V7.2 (international OpenGATE collaboration). The GATE software is based on Geant4 toolkit.

Results: The absorbed dose results for 6-13 weeks gestation to embryo/fetus from maternal uterus, thyroid, and ovary were 0.146-0.272, 0.087-0.097, and 0.020-0.043 mGy/(MBq.h), respectively. The absorbed dose for the embryo/fetus from the ovary also increased with the age of the fetus, while it decreased with the age of the embryo/fetus from the uterus. Furthermore, there was no significant change with increasing embryo/fetus age in absorbed doses by embryo/fetus from maternal thyroid gland. Moreover, self-absorbed dose range to fetus from its thyroid was 0.233-0.408 mGy/(MBq.h), which is higher than absorbed dose from maternal organs. The most interesting result was fetus’s absorbed dose from its thyroid gland after the 9^th week, which reached the maximum in 9^th weeks and decreased with increasing fetal age until the 13^th week.

Conclusion: The results showed that the absorbed dose by the fetus in the fetal thyroid gland, as well as the absorbed dose by the maternal organs, contributed significantly to the total dose of [¹³¹I]NaI absorbed by the fetus. Malformation and damages to central nervous system are the main risks of radiation to embryo/fetus after ¹³¹I-therapy that must be considered for continuing the pregnancy. Based on calculated factors, physicians or physicists can estimate the absorbed dose to embryo/fetus in pregnant cases by ¹³¹I-therapy with greater accuracy than before for a possible decision on termination of pregnancy.