X rays are ionizing radiation, and doses arc expressed in rad or in Gray (Gy), where 1 Gray = 100 rads = 100000 mrad. Only the radiation dose that reaches the organ in question - in this case, the uterus, ovaries or the fetus - is relevant. In the target organism, i.e. the embryo, the actually determined dose is expressed in rem or Sievert (Sv), where 1 Sievert = 100 rem = 100 000 mrem. Assuming that the emitted dose to the organ corresponds to the dose delivered to the embryo, 1 Sv corresponds to 1 Gy.
The cardinal manifestations of intrauterine radiation effects in humans are growth retardation and defects of the central nervous system (CNS) - mainly microcephaly with mental retardation, and eye malformations. These effects are a function of the dose administered and of the stage of development of the embryo. Embryo/fetai death may also occur during the first 5 days after conception (i.e. in the "all-or-none period"); the lowest lethal dose is 10 rads (0.1 Gy). During embryogenesis, the lowest lethal dose for the embryo increases to 25-50 rads and later to more than 100 rads (lGy) (Brent 1999). Severe CNS malformations are to be expected with exposures above 20 rads during early gestation (18-36 days after conception). Microcephaly and mental retardation were observed only after exposures above 20 rads between weeks 8 and 15 after conception. The conclusion from most studies is that for doses lower than 0.05 Gy (i.e. 5 rads) there is no significant increase of the malformation rate in humans, and the risk clearly is increased above 20-50 rads (Brent 1999. Sternberg 1973). A common and important finding is the absence of visceral, limb or other malformations unless there is growth retardation, microcephaly or congenital malformation of the brain, or visible eye malformations such as microphthalmia, optic atrophy, or cataract. De Santis (2005A) and Brent (1999) reviewed the literature and concluded that inadvertent exposure to ionizing radiations due to diagnostic procedures during pregnancy doesn't increase the background risk of congenital anomalies. It has nevertheless been suggested by different authors that pregnancy exposure to ionizing radiations increases the risk of low birth weight (Hamilton 1984). Hujoel (2004) performed a case-control study and concluded that dental X-rays during pregnancy were a risk factor for low birth weight. They suggest as an interpretation that an alteration of the maternal hypothalamus-hypophysis-thyroid axis might affect birth weight, with a threshold dose-effect of 0.4 mGy at the thyroid. De Santis (2005B) studied the pregnancy outcome of 224 women who underwent thyroid irradiation as a diagnostic procedure during the first trimester, and also found a moderate reduction in the birth weight with a dose threshold of 0.4-0.8 mGy at the thyroid level. These studies concerning the thyroid are considered to be inconsistent, because hypothyroidism cannot be produced with 0.4 mGy (Brent 2005, Boicc 2004).
The mutagenic or transplacental carcinogenetic risk of ionizing radiation is more difficult to evaluate than the teratogenic risk. Mutagenic effects are stochastic events, and no threshold can be established for this kind of risk. Point mutations often occur spontaneously. Estimations have been made that a dose of 100-200 rads may induce a doubling of the point mutation rate (Brent 1999, Neel 1999). On the one hand, a doubling of the mutation rate of a given gene does not mean a doubling of the disease frequency; on the other hand, insufficient knowledge regarding the possible effects on later generations prevents definition of safety limit values of exposures for a population (Brent 1999). The dose capable of increasing the cancer risk, especially the risk of childhood leukemia following prenatal exposure, is also not well established. In a case-control study on neuroblastoma, Patton (2004) found no consistent exposure -response gradient based upon the number of maternal or paternal medical radiation examinations. In a study of twin pregnancies conducted by Harvey (1985), it was concluded that a prenatal dose of0,01Sv(l rem) might multiply the risk of childhood leukemia by a factor of 2.4. Wakcford (2003) concluded, from the Oxford Survey of Childhood Cancers, that doses to the fetus in utero of the order of 10 mSv discernibly increases the risk of childhood cancer.
However, uncertainties in risk estimates are such that it is difficult to conclude reliably from these epidemiological data what the level of risk at these low doses might be, beyond the inference that the risk is not zero or grossly underestimated. Other authors assume that there is no increased risk for the embryo with exposures of 0.02-0.05 Sv (Boicc 1999) because the increased lifetime risk would be very small - the current risk is 18 per 100 000 and, based upon this extrapolation, would become 18.024 per 100000.
The usual conventional X-ray examinations, including examination of the lower abdomen, all give a dose of less than 5 rem. In most cases, in a single X-ray of the abdominal, pelvic, and lumbar spine region (without shielding of the uterus), the dose will be well under 200 mrem, provided that examinations are conducted with current and correctly adjusted equipment. Longer screening times, as used in intestinal explorations or urographies, can lead to a dose to the uterus of 2 rem. Endoscopic retrograde cholangio-pancrcatography (ERCP) was evaluated in 17 pregnant women (Kahaleh 2004), and the mean fetal radiation exposure was 40-46 mrad (range 1-180 rnrad). Computerized tomography (CT scan) delivers higher doses; however, these mostly stay below 5 rem. The secondary irradiation owing to examinations of other body regions, such as the upper abdomen, thorax, extremities or teeth, is negligible, because the doses delivered to the uterus lie well below 10 mrem - even as low as 1 mrem.
Recommendation. As a woman may not be aware of her pregnancy at the beginning, radiologists should not rely on a negative answer from a woman asked about a possible pregnancy before a radiological exploration. Any necessary X-ray examination of the lower abdomen should be performed only during the first half of the menstrual cycle. If X-ray examinations are indispensable during pregnancy, only the most modern devices should be used, with optimal protection of the uterus. In any case of X-ray examination during pregnancy that (inadvertently) includes the uterus, the uterine dose should be assessed and documented; this is especially important in cases of X-ray screening for more than 15 seconds, or a CT scan.
Neither radiographs outside the genital region and the uterus, nor the usual X-ray examinations (including standard CT scan) inadvertently including the pregnant uterus, require termination of pregnancy or any additional diagnostic procedures.
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