Dependence of the RBE of densely ionizing radiation from radiosensitivity and stage of cell growth
The analysis of the patterns of densely ionizing radiation RBE display is an urgent problem for the radiological substantiation of the hadron therapy of human tumors that is being actively introduced into practice. To experimentally study the dependence of RBE of densely ionizing radiation on radiosensitivity and the stage of cell growth; confirm the participation of cell repair processes in the formation of alpha particles RBE. The experiments used wild-type haploid and diploid yeast cells Saccharomyces cerevisiae and their radiosensitive mutants, which are incapable of recovering from radiation damage induced by sparsely and densely ionizing radiation. Cells were irradiated in stationary and logarithmic growth stages. It was shown that for haploid yeast cells exposed to <sup>60</sup>Co g-rays and <sup>239</sup>Pu a-particles in the stationary stage of growth RBE was 2.3 and did not depend on the dose of ionizing radiation. On the contrary, at the stage of cell division, the RBE values increased from 1.9 to 6 with dose increase. When diploid cells are irradiated in the stationary stage of growth, RBE=5.4 and almost did not depend on dose while at the stage of cell reproduction RBE of a-particles sharply increased from 4.5 to 8.7 with dose decrease. For radiosensitive mutants unable to recover from DNA damage, the RBE of a-particles weakly depended on cell ploidy and decreased slightly upon irradiation of dividing cells (RBE=1.6) as compared to irradiation of cells in the stationary growth stage (RBE=2.2). It is concluded that the results obtained are consistent with the point of view that the RBE of densely ionizing radiation is determined not only by the probability of the primary damage production at the physicochemical stage of radiation interaction with cellular structures, but also by the cell ability to recover from sublethal and potentially lethal damage at biochemical stage of their fixation. It is stressed the need to take into account the obtained variability of RBE values of high LET radiation at the interpretation of experiments with cultured mammalian cells. Since the processes of a-particles interaction with living matter are qualitatively identical to the interaction of protons produced in tissues during neutron irradiation, the results obtained here should be taken into account when analyzing the effectiveness of antitumor radiation therapy of hadron radiation - neutrons, protons and other heavy charged particles.