Ionizing radiation is known to cause harm in mammalian organisms. Deleterious effects of radiation include carcinogenicity, mutagenicity and organ system toxicity. As general rule, the sensitivity of a tissue to radiation is directly proportional to its rate of proliferation. Therefore, one could infer that the human fetus, because of its rapid progression from a single cell to a formed organism in nine months, is more sensitive to radiation than the adult. This inference is supported by the results of experiments in animal models, and experience with human populations that have been exposed to very high doses of radiation (atomic bombing victims). In humans, the major deleterious effects on the fetus include fetal wastage (miscarriage), teratogenicity (birth defects), mental retardation, intrauterine growth retardation and the induction of cancers (such as leukemia) that appear in childhood. Birth defects and mental retardation are the adverse effects which are of the most immediate concern for expectant mothers. Fortunately, not all exposures to ionizing radiation result in these outcomes. The risk to the fetus is a function of (a) gestational age at exposure and (b) the radiation dose.
At the level of most diagnostic procedures ( fetal dose < 10 rem), little data in humans is available. However, some qualitative observations regarding fetal risk can be made.
Risk Related to Gestational Age
Early Gestation / First Trimester -- At this point, the rate of fetal growth is very rapid and the fetus, as an organism, is at its most radiation-sensitive stage if fetal demise is taken as an end-point. The incidence of fetal wastage consequential to radiation exposure at this stage of gestation is not known, since (a) many women were never aware they were pregnant at the time of the exposure or miscarriage, and (b) the "background" rate of miscarriage is believed to be high (25 - 50 percent of conceptions). It is believed that radiation injury during early gestation is an "all-or-nothing" effect.
Second Trimester -- During this period, the overall growth rate of the fetus has slowed. However, the major organ systems are beginning to differentiate. From a standpoint of future development, the fetus is in its most sensitive stage. The incidence of gross congenital malformations and mental retardation are dose-related and appear to have thresholds; i.e. doses below which the incidence above "background" is not elevated.
Third Trimester -- Irradiation during this period may deplete cell populations at very high doses (over 50 rem), but will not result in gross organ malformations.
Risk Related to Radiation Dose
The risk of deleterious effects increases with increasing dose. The nature of this dependence, i.e. the shapes of the dose-response curves for humans in the low-dose range (under 50 rem), is controversial. For some prenatal irradiation effects, there is epidemiological basis for the existence of threshold doses. For others, such as childhood cancer induction, the existence of a threshold is not clear-cut. Despite these uncertainties in the dose-effect relationship, some broad generalizations based on fetal dose ranges may be made.
Fetal Dose Less Than 1,000 millirem -- There is no evidence supporting the increased incidence of any deleterious developmental effects on the fetus at diagnostic doses within this range.
Fetal Dose between 1,000 millirem and 10,000 millirem -- The additional risk of gross congenital malformations, mental retardation, intrauterine growth retardation and childhood cancer is believed to be low compared to to the baseline risk. However, the lower limits (in terms of statistical confidence intervals around the mean) for threshold doses for some studies, especially those related to cancer induction, fall within this range.
Fetal Dose Exceeding Than 10,000 millirem -- The lower limits (in terms of statistical confidence intervals) for threshold doses for effects such as mental retardation and diminished IQ and school performance fall within this range. Overall, exposure at levels exceeding 10 rem could be expected to result in a dose-related increased risk for deleterious effects. For example, the lower limit (95% confidence interval) for the threshold for mental retardation is about 15 rem, which an expectation value of about 30 rem.
Counseling the Pregnant Patient Exposed to Ionizing Radiation
Due to the complexity of the issues surrounding fetal irradiation, there is no "standard" or predetermined advice that can be given to the expectant patient. However, it is possible to assist the patient in assessing the implications of the exposure if a systematic evaluation of the risk is performed. According to Dr. Robert Brent, the following parameters should be considered in the evaluation:
- Gestational age at the time of exposure
- Menstrual history
- History of previous pregnancies, including a history of congenital malformations
- Other potentially harmful environmental factors (malnutrition, smoking, alcohol / drugs, etc.)
- Maternal / paternal age
- Calculation of fetal exposure using dose reconstruction techniques
- Attitude of the mother toward the pregnancy.
In any event, it is important not to defer medically necessary studies with anticipated fetal doses of less than 5,000 millirem based solely on a concern for causing adverse fetal effects.