during gestation at which the exposure occurred and for the particular abnormality considered. Threshold doses for some abnormalities have been demonstrated, but such thresholds vary for different abnormalities. Recent data from the Japanese atomic bomb studies demonstrate measurable teratogenic damage (e.g., small head size associated with mental retardation) at doses of 10-19 rad (kerma) (NAS-BEIR, 1980). Decreasing the dose rate generally leads to a decrease in the developmental effects. EFFECTS ON THE EYE Cataracts. The most important response of the human eye to chronic radiation exposures is the production of cataracts resulting in impairment or loss of vision. The committee believes that cataract induction in space workers may be a problem, depending in part on the fluence and the RBE of the HZE particles. Impairment of vision in man can occur after single and fractionated exposure to low- and high-LET radiations (Britten et al., 1966). The response, including the length of the latent period and the severity of the effect, is dose-dependent above a threshold level. The dose-response curve for low-LET radiation-induced cataracts is highly sigmoid (Merriam and Focht, 1962). Above the threshold, the incidence of lens changes increases non-linearly with dose, the time to appearance is shortened, and the number of opacities that progress to cataracts with visual impairment increases. Opacities occur with doses from 200 to 500 rad, but do not progress to cataracts. Opacities may occur with 400 rad when the dose is fractionated over a period of three weeks and with 600 rad if the fractionation extends beyond three months. For low-LET radiations, protraction of the exposure is thought to reduce the effect on the lens less than for some other tissues. Workers exposed to fast neutrons from cyclotrons have developed cataracts (Abelson and Kruger, 1949); however, precise dose levels are not known, and a reliable quality factor cannot be derived. In radiotherapy patients exposed to 14-MeV cyclotron neutrons, no change in the lens was seen following 80 rad in 12 fractions. However, slight permanent vision loss was found after 220 rad (Roth et al., 1976). Despite a knowledge of the neutron dose in the Japanese atomic bomb survivors, the neutron dose effect on the induction of cataracts is not known. Therefore, the RBE for cataract induction in man remains unknown, although the RBE will be dependent on the dose. Although rodents are considered much more susceptible than humans to radiation-induced cataracts, rodent studies provide data on LET-RBE relationships that may obtain for human risk estimation. The mouse lens is sensitive to high-LET neutrons; the RBE at doses less than 10 rad could approach 100 or greater when "fleck" opacities are scored. The human lens would also be expected to be sensitive to densely ionizing radiations, such as neutrons or HZE particles (Merriam et al.,
RkJQdWJsaXNoZXIy MTU5NjU0Mg==