Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

a specific organ system (the so-called critical period) exists when the radiationexposure is received during the period of peak differentiation of that system. Thismay not always be the case, however, because damage can occur to adjacent tissue,which has a negative effect on a developing organ system. Some anomaliesmay have more than one critical period. For example, cataract formation hasbeen shown to have three critical periods in mice. Figure 25-19 shows the criticalperiods in human fetal development for various radiation-induced birthdefects.The only organ system (in humans or laboratory rodents) that has shown anassociation between malformations and low-LET radiation doses less than 250mGy (25 rad) is the eNS. Embryos exposed early in organogenesis exhibit thegreatest intrauterine growth retardation, presumably because of cell depletion. Inutero exposure to doses greater than 100 mGy (10 rad) of mixed neutron andgamma radiation from the Hiroshima atomic bomb resulted in a significantincrease in the incidence of microcephaly. Low doses of x-rays have also been shownto produce growth retardation and behavioral defects.In general, radiation-induced teratogenic effects are less common in humansthan in animals. This is primarily because in humans a smaller fraction of the gestationalperiod is taken up by organogenesis (about 1/15 of the total, compared withFIGURE 25-19. Critical periods for radiation-induced birth defects in humans. Data wereobtained from children who were exposed in utero as a result of medical radiation treatments oftheir mothers. Fetal doses were in excess of 2.5 Gy (250 rad). Most children had more than oneanomaly, and mental retardation was usually associated with microcephaly when exposureoccurred during this period of gestation. No significant increase in any of the effects listed herehave been shown to occur in humans before the 7th week of gestation. The radiation-inducedrisks reported before the 7th week of gestation are from animal data. The broader lines indicateincreased probability of effect. (Data from Dekaban AS. Abnormalities in children exposed to x-irradiation during various stages of gestation: tentative time table of radiation injury to humanfetus. J Nucl Med 1968;9:471-477.)