A42 Stochastic effects

If a cell is not directly killed by radiation but somehow modified, the outcome will be different from those included among deterministic effects. In vitro cellular researches show that damage from radiation to deoxyribonucleic acid (DNA) gives rise to most of the detrimental effects. There are two mechanisms by which radiation may damage DNA: direct or indirect interaction. In the first case ionizing radiation directly damages a gene, in the second case radiation produces active chemical radicals near the DNA. The diffusing radicals may interact with DNA and induce chemical changes. Very efficient mechanisms exist (enzyme actions) to repair DNA, whatever the cause of harm. If only one of the two symmetric strands forming the DNA is damaged, the use of information on the other strand makes the repair process highly probable and successful, though it is not always error-free. If both strands are damaged at the same location, information is lost forever: the repair process is more difficult and genetic changes are likely. Such changes are defined as genetic mutations. The very nature of this process of damage/repair gives rise to effects that are random and statistical, and therefore are defined as stochastic. Stochastic effects can be somatic (i.e., cancer induction), that is they occur on the exposed individual, or hereditary: damaged cells are those whose function is to transmit genetic information to offspring. As there is no evidence that below a certain dose the repair process is totally effective, differently from deterministic effects, there is no threshold in this case [UNSCEAR, 1993].

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