[Different levels of radiation protection in tumor cell population]

It is postulated the presence of various systems of protection in tumor cell population. This systems have different mechanisms and are induced by the level of damage. From the analysis of the literature and own experiments the author maintains that there are not less than three levels of tumor cell population protection. 1. The radiation induced protection system is engaged after relative low radiation doses (1-4 Gy). Basic mechanism is stimulation of intracell DNA repair. 2. Repopulation is restore of number of cells, which were in a radioresistant state at the moment of irradiation. This second protection level is induced by radiation with doses 4.5-10 Gy. 3. Nonspecific protection mechanism opposing hard destruction effects functions at cell population level after irradiation with doses above 10 Gy. As a result of radioinduced fusion of lethally damaged cells, and also as a result of generation of polyploid and multinuclear cells and other reorganization processes aroused by radiation in high doses, viable somatic cell hybrids and genetically changed cells may be formed. In this case repopulation may occur owing to irradiated tumor cell progeny proliferation appeared after indicated processes.