[Radiology and radiation protection in dental medicine. 1: Biological effects resulting from exposure to ionizing radiation].

The different types of biological effects produced by ionizing radiations are reviewed. Among the early effects following an acute exposure, the bone marrow syndrome and the intestinal syndrome are briefly described. They are unlikely to appear in the current conditions encountered in dental radiology. As far as skin exposure is concerned (e.g. fingers, hands), acute reactions (moist desquamation, ulceration, etc.) require high doses which are encountered only in accidental conditions. However, chronic dermatitis after repeated low dose exposure is one of the major risks in dental radiology (even without previous acute lesions), if the recommendations of radioprotection are not followed. The danger is ultimately the transformation into a radio-induced skin cancer. Among the late effects, after chronic exposure at low doses, cancer induction and the genetic effects are the most important. The risk coefficient for cancer induction is expressed by the number of lethal cancers induced per unit dose. In his last report (1988), the UNSCEAR assessed the risk coefficient between 4.5 and 7% for an absorbed dose of 1 Gy (low-LET radiation, high dose delivered at high dose rate). A low doses and low dose rates (X-and gamma rays), the UNSCEAR recommends to reduce the above risk coefficients by a factor between 2 and 10. The risk coefficient at low doses is difficult to evaluate since the rate of radio-induced cancers is small compared to the spontaneous cancer incidence. The risk of a severe genetic effect is about 1/3-1/4 of the risk of lethality from radio-induced cancer. Finally, the characteristics of the stochastic and non-stochastic effects are described. The acute effects, the late radiodermatitis and the cataract are examples of non-stochastic effects, while cancer induction and the genetic effect are stochastic effects. In the Appendix, the most important quantities and units, used in radiation protection, are presented.