A.2.3 Radiation Sensitive Subgroups
There are several groups of individuals who are significantly more sensitive than
average, and these may need special consideration:
• Infants and children;
• Individuals with genetically based hypersensitivity to ionizing radiation;
• The developing embryo or fetus of a pregnant woman.
The risk for radiation-induced cancer increases with decreasing age at time of
exposure (ICRP, 1991). Very roughly, a neonate is about three times more sensitive than a 25 year old adult.
There is reasonable evidence that three to five percent of the population is
significantly more sensitive to ionizing radiation than average (e.g., Schultheiss et al., 1995), and it has long been speculated that this hypersensitivity is genetically based.
It is important to note that there is not as yet direct evidence for human subgroups that have increased susceptibility to radiation-induced cancer, although there is suggestive evidence from oncogenic transformation studies of ATM heterozygote mouse embryos (Smilenov et al., 2001). However, it is too early in the scientific research on this hypersensitivity to be able to take into account the significance of radiosensitive subgroups (ICRP, 1998), and risk estimates for the general population might currently be sufficiently stringent to protect these subgroups.
The developing embryo and fetus are especially sensitive to ionizing radiation.
Risk estimates for congenital malformations and functional impairment after in utero exposure are typically an order of magnitude higher than those for radiation-induced cancer. It may well be that deterministic endpoints such as congenital malformations have a threshold in dose, below which the risk is zero. Visual inspection of the data often suggests that a threshold may exist, but little statistical support is available, except in the case of mental retardation (Otake and Schull, 1998).