DCA05DR Dental Radiation Health, Safety, and Protection, 3rd Edition _______________________________
Two basic types of tissues make up the body: somatic and genetic . Somatic tissues are all the tissues of the body except those involved in reproduction. The effects of radiation exposure on somatic tissues follow a nonlinear threshold dose-response model. This means that above a certain threshold or dose level the effects are observed clinically, and below the threshold no damage is visible. As the dose increases beyond the thresh- old, more severe damage is produced and observed clinically. Genetic tissues include reproductive and embryonic tissues. Whereas radiation injury to somatic tissues affects only the exposed person, injury to genetic tissues may impact future generations. Genetic or heritable effects may result in mutations or effects on the embryo that are passed on but may not be expressed for years. In contrast to the effects of exposures to somatic tissue, the effects on genetic tissue follow an LNT dose- response model, in which no dose is considered safe. However, as genetic exposures increase, so does the degree of mutation. As demonstrated by genetic exposures, some cells and tissues are more sensitive to the effects of radiation than others. The law of Bergonié and Tribondeau describes radiosensitivity as a function of the number of undifferentiated cells in the tissue, their mitotic activity, and the duration spent in their devel- opment (Honjo & Ichinohe, 2020). Radiosensitive tissues exhibit particular characteristics: They have a high rate of cell division (mitosis) (Honjo & Ichinohe, 2020; Radiology Key, 2021), a high rate of cellular metabolism (Read et al, 2022;
Radiology Key, 2021), and a primitive or immature nature (Radiology Key, 2021), and they are nonspecialized (Ministry of the Environment, 2022; Geha, 2022). Figure 2 provides examples of the cells and tissues that have high, intermediate, and low sensitivity to ionizing radiation. Finally, the volume of tissue exposed to radiation influences the effects of radiation. The body can better tolerate high doses of radiation delivered to a localized area than to the whole body. Radiation oncology, which fractionates a high total dose into multiple small doses to allow for tissue repair and recovery between doses, employs this knowledge. By contrast, high doses delivered to the whole body are debilitating, if not fatal. For example, the nuclear reactor meltdown at Chernobyl, Ukraine, produced lethal whole-body exposures to firemen and cleanup workers (National Cancer Institute, 2022).
SHORT- AND LONG-TERM EFFECTS OF RADIATION EXPOSURE
For the most part, radiation effects are not observed immedi- ately. The delay, or time lag, between the exposure to radiation and the first clinical signs of biologic damage is known as the latent period . The length of the latent period is dependent upon the total dose of radiation received and the rate at which the dose was delivered. In general terms, the greater the amount of radiation received and the faster its delivery, the shorter the latent period.
RELATIVE RADIOSENSITIVITY OF CELLS AND TISSUES
Source: Radiation Key, 2021; Ministry of the Environment, 2022.
Figure 2
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