The harmful deterministic efects of ionizing radiation have been extensively studied for cell damage with resultant disordered embryogenesis. These have been assessed in animal models, as well as in Japanese atomic bomb survivors and the Oxford Survey of Childhood Cancers (Sorahan, 1995). Additional sources have confirmed prior observations and provided more information (Groen, 2012). One is a 2003 International Commission on Radiological Protection publication that describes biological fetal efects from prenatal irradiation. Another is the Biological Efects of Ionizing Radiation-BEIR II Phase 2 report of the National Research Council (2006), which discusses health risks from exposure to low levels of ionizing radiation.
In the mouse model, the lethality risk is highest during the preimplantation period-up to 10 days postconception (Kanter, 2014). This is likely due to blastomere destruction caused by chromosomal damage (Hall, 1991). During organogenesis, high-dose radiation-1 Gy or 100 rad-is more likely to cause malformations and growth restriction and less likely to have lethal efects in the mouse. Studies of brain development suggest efects on neuronal development and a window of cortical sensitivity in early and midfetal periods. That said, acute lowdose ionizing radiation appears to have no deleterious efects (Howell,r2013).
Data on adverse human efects of high-dose ionizing radiation mostly derive from the atomic bomb survivors of Hiroshima and Nagasaki (Greskovich, 2000; Otake, 1987). The International Commission on Radiological Protection (2003) confirmed initial studies showing that the increased risk of severe mental retardation was greatest between 8 and 15 weeks' gestation.
There may be a lower-threshold dose of 0.3 Gy (30 rad), which is a range similar to the window of cortical sensitivity in the mouse model discussed earlier. he mean decrease in intelligence quo tient (IQ) scores was 25 points per Gy or 100 rad. here appears to be linear dose response, but it is not clear whether there is a threshold dose. Most estimates err on the conservative side by assuming a linear nonthreshold hypothesis. In a study of fetuses exposed to low radiation doses, Choi and colleagues (2012) did not find an increased risk for congenital anomalies.
Finally, an increased risk of mental retardation in humans < 8 weeks' or > 25 weeks' gestation has not been documented, even with doses exceeding 0.5 Gy or 50 rad (International
Commission on Radiological Protection, 2003). Reports have described high-dose radiation used to treat women for malig nancy, menorrhagia, and uterine myomas. Dekaban (1r968) described 22 infants with microcephaly, mental retardation, or both following exposure in the first half of pregnancy to an estimated 2.5 Gy or 250 rad.
Summary of Fetal Radiation Exposure
From 8 to 15 weeks, the fetus is most susceptible to radiation induced mental retardation. It has not been resolved whether this is a threshold or nonthreshold linear function of dose. he
Committee on Biological Efects (1990) estimates the risk of severe mental retardation to be as low as 4 percent for 0.1 Gy (10 rad) and as high as 60 percent for 1.5 Gy (150 rad). But recall that these doses are 2 to 100 times higher than those considered maximal from diagnostic radiation. Importantly, cumulative doses from multiple procedures may reach the harmful range, especially at 8 to 15 weeks' gestation. At 16 to 25 weeks, the risk is less. And again, there is no proven risk before 8 weeks or after 25 weeks.