[Main Principles of Radiobiology]

During all the history of the development of radiation biology a problem of "energy paradox"--low consumption of energy of ionizing and non-ionizing radiation in realization of irradiation effect--has been in the focus. The first principle, which contributed much to quantitative concepts of radiation biology, is a hit principle. The hit principle, as is well known, is based on physical properties of ionizing radiations: their discontinuity, quantization and probabilistic distribution in space. Hits, i.e. acts of energy interaction with substance elements, do not depend on each other and are obeyed to Poisson distribution. The other well-known principle--a target principle--is based on the understanding that a living system has some peculiarities: a structure of elements as well as their functions are heterogeneous, unequal and differ in response to the same hits. Along with a unique DNA macromolecule, a critical target structure, biological membranes (BM) with their barrier-matrix, energy and regulatory functions, which make a basis of living processes, are also can be considered as a sensitive target structure. One more principle--a principle of amplification of primary radiation lesions in critical target structures is based on the radiation post-effect, a well-known phenomenon in radiation biology. The fourth principle is a principle of target damage recovery (regulations of cell homeostasis) that means a system response to irradiation involving mechanisms of protection and reparation of lesions in DNA and BM. The progress in molecular biology and radiation biophysics achieved for the last two decades provided an especially powerful impetus to the development of those principles, which are based on the analysis of the radio-biological effects developing in time. The main principles of radiation biology consider peculiarities of physical and biological action of ionizing radiation.