Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

1/3 for mice). Nevertheless, development of the CNS in humans takes place over amuch longer gestational interval than in experimental animals, and therefore theCNS is more likely to be a target for radiation induced-damage. An important distinguishingcharacteristic of teratogenic effects in humans is the concomitant effectson the CNS and/or fetal growth. All cases of human in utero irradiation that haveresulted in gross malformations have been accompanied by CNS abnormalities orgrowth retardation or both.The response of each organ to the induction of radiation-induced malformationsis unique. Such factors as gestational age; radiation quantity, quality, and doserate; oxygen tension; the cell type undergoing differentiation and its relationship tosurrounding tissues; and other factors influence the outcome.Fetal Growth StageThe fetal growth stage in humans begins after the end of major organogenesis (day45) and continues until term. During this period the incidence of radiationinducedprenatal death and congenital anomalies is, for the most part, negligible.Anomalies of the nervous system and sense organs are the primary radiationinducedabnormalities observed during this period, which coincides with their relativegrowth and development. Much of the damage induced at the fetal growthstage may not be manifested until later in life as behavioral alterations or reducedintelligence (e.g., IQ).Two groups that have been studied for the effects of in utero irradiation are the childrenof the atomic-bomb survivors and children whose mothers received medicalirradiation (diagnostic and/or therapeutic) during pregnancy. The predominanteffects that were observed included microcephaly and mental and growth retardation.Eye, genital, and skeletal abnormalities occurred less frequently. Excluding thementally retarded, individuals exposed in utero at Hiroshima and Nagasaki betweenthe 8th to 25th week after conception period demonstrated poorer IQ scores andschool performance than did unexposed children. No such effect was seen in thoseexposed before the 8th week or after the 25th week. The decrease in IQ was dosedependent, with an apparent threshold below 250 mGy (25 rad). The greatest sensitivityfor radiation-induced mental retardation is seen between the 8th and 15thweeks, during which the risk is approximately 1/250 or 0.40% per Gy (IOO rad) tothe fetus.Microcephaly was observed in children exposed in utero at the time of theatomic bomb detonation. For those exposed before the 18th gestational week, theincidence of microcephaly was proportional to dose, up to 1.5 Gy (I50 rad), abovewhich the decreased incidence was presumably due to the increase in fetal mortality.The incidence of microcephaly (normally ~3%) was increased when exposuresoccurred during the first or second trimester. No excess was found with exposure inthe third trimester, regardless of dose. The incidence of microcephaly in the fetaldose range of 100 to 490 mGy (IO to 49 rad) was approximately 19% and 6% forthe first and second trimester, respectively. At fetal doses greater than 1 Gy (IOOrad), the incidence of microcephaly rose to approximately 83% and 42%, for exposuresin the first and second trimester, respectively.