When a pregnant female is exposed to ionizing radiation special consideration must be given to the radiosensitive unborn child. 1 15 Fetal cells are largely undifferentiated and highly proliferative and thus have increased radiosensitivity depending on the phase of gestation ( Fig 199.-.3.). At 0 to 2 weeks postconception, there is an all-or-none phenomenon. Irradiation during this time usually results in death with resorption of the conceptus or no observable damage. Ihis phenomenon is a result of the pluripotential of the early blastomeres that allows injured cells to be replaced by remaining cells when the damage is not extensive. After two weeks gestation, organogenesis begins and the embryo is at risk of congenital malformations. Ihe risk of injury is greatest for the particular organ system that is under development at the time of radiation exposure. After seven weeks, the fetal period begins. Major organogenesis is complete, with the exception of the CNS. Ihe CNS has continued susceptibility to radiation injury during the early fetal period. 15
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FIG. 199-3. The various adverse effects associated with radiation and their relative incidence at different stages of gestation. (Reprinted with permission from Medical Effects of Ionizing Radiation, 2d ed. Philadelphia, WB Saunders, 1995.)
Data of radiation-induced anomalies in humans has been derived from Japanese atomic bomb survivors who received radiation exposure in utero. The most common injuries are related to the CNS, particularly microcephaly and mental retardation. Other malformations, such as growth retardation and ocular defects, have been reported less frequently. Radiation exposure during 8 to 15 weeks' gestation correlates with the most significant CNS injury. This increased radiosensitivity is due to the extensive proliferation and migration of neuronal cells at this time.15 Both human and animal studies indicate that a minimum fetal exposure of 0.1 to 0.2 Gy (10 to 20 rad) is required for injury.15 Official agencies, including the National Council on Radiation Protection (NCRP), recommend that the dose to an unborn child be limited to 500 millirem (5 milliSv).16 Below this limit, the risks to the unborn child exposed with radiation do not exceed the baseline rate of birth defects seen in infants not exposed to radiation.17
If a fetal dose is above 500 millirem (5 milliSv), particularly during the vulnerable period of 8 to 15 weeks' gestation, risks such as CNS damage or growth defects, must be considered. In such a case, a physician with expertise in radiation injury should be consulted to provide counseling to the expecting parents.
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