Biophysical mechanisms of the ionizing radiation effect and their importance in medicine

The lethal effect of various kinds of radiation is usually characterized by the survival curve, or the S(D) function, expressing the fraction of cells persisting after the application of dose D. Detailed analysis based on data obtained in vivo demonstrates that the survival curves can generally have a rather complex shape. The principal damage responsible for the lethal effect consists in the so-called double fractures of chromosomal DNA molecules, resulting in combinations of adjacent single fractures in the two chains of the molecule. Damage of all the other systems can be repaired by the cell rather easily. Thus, the shape of the survival curve is generally governed by the formation of the double fractions as well as by the rate of the repair process. In the radiotherapy of tumor diseases, the tumor focus is destructed while sparing, as far as possible, the healthy cells from which the proliferation and restitution of the tissue should proceed. This can be achieved by fractionation of the total dose. The fractionation regime is characterized by the total dose D, total exposure T and the number of fractions. (Z.S.). 4 refs