Cataractogenesis
The lens of the eye is sensitive to radiation and develops cataracts on irra-
diation with ionizing radiations. The incidence of radiation-induced
cataracts is a deterministic effect and depends on the dose given. A dose of
10 to 30 rad (10 to 30 cGy) is required to produce cataracts in mice, whereas
a threshold dose of about 200 rad (200 cGy) is needed to produce cataracts
in humans in a single exposure. A larger dose is required if the dose is given
in fractions, and high-LET radiations almost double the incidence of
cataractogenesis. A minimum of 1 year is needed for the latent period of
cataractogenesis.
Radiation Damage to Embryo and Fetus
The developing mammalian embryo is extremely sensitive to ionizing radi-
ations, because many cells are differentiating at this stage. The degree of
damage depends on the developmental stage of the embryo, the dose, and
the dose rate. The entire fetal development is divided into three general
stages: (1) preimplantation, a period of about 8 to 10 days between fertil-
ization of the egg and its attachment to the uterine wall; (2) major organo-
genesis, a period of about 2 to 6 weeks, when major organs are developed;
and (3) the fetal stage, the remainder of the pregnancy period, when the
organs of the fetus grow further to enable the mammal to survive after
birth.
The embryo in the preimplantation stage is most sensitive to ionizing
radiations and mostly encounters prenatal embryonic death as a result of
radiation exposure. In some species, a dose as low as 5 to 15 rad (5 to
15 cGy) is suffcient to cause deleterious effects on the embryo. At a dose of
200 rad (200 cGy) in the preimplantation stage, embryonic death is certain.
Almost all embryos that survive the radiation exposure grow normally in
utero and afterward, with the exception of a few that develop abnormali-
ties later.
During the period of major organogenesis, embryos exposed to ionizing
radiations develop abnormalities mostly related to the central nervous
system (CNS) and bone. These abnormalities are too severe for the fetus
to survive and ultimately result in neonatal death. At an exposure of 200
rad (200 cGy) to mouse embryos during this period, almost 70% of the
embryos later experienced neonatal death. Growth retardation also is noted
at doses above 100 rad (100 cGy). Often it is suggested that a therapeutic
abortion should be considered if an embryo receives ~10 rad (10 cGy)
during the first 6 weeks after conception.
During the fetal period, however, comparable doses do not cause any
abnormality or neonatal death, because fetal cells are more radioresistant
than embryonic cells. Relatively higher doses are needed to cause death in
this period. A few cases of growth retardation have been noted. In utero
irradiation with a dose of 1 to 2 rad (1 to 2 cGy) may increase the risk of
Long-Term Effects of Radiation 255