Track structure analysis of ultrasoft X-rays compared to high- and low-LET radiations.

Monte-Carlo track structure simulations of ultrasoft X-rays, and of selected low- and high-LET radiations for comparison, have been used to obtain statistically valid frequency distributions of energy deposition in small subcellular targets which resemble the dimensions of short segments of DNA, nucleosomes and short segments of chromatin fibre. It is found that in all cases large numbers (approximately 10(3] of direct energy deposition events occur in these targets in a single mammalian cell irradiated with 1 Gy of any of these radiations. In almost all cases the numbers of energy depositions of substantial size (say, approximately greater than 100 eV in a DNA segment, approximately greater than 300 eV in a nucleosome or approximately greater than 800 eV in a segment of chromatin fibre) are also quite large, being approximately 10 to 100 per cell per Gy. It seems clear therefore that the direct effects of radiation on macromolecules must be considered in assessing the biological effects of any ionizing radiations on mammalian cells. The calculations also show that high-LET radiations can produce uniquely large energy depositions in the targets, such as are virtually unachievable by any of the other radiations; this allows the possibility of unique biochemical and cellular damage by high-LET radiations. At any realistic dose for mammalian cells, virtually all the energy depositions in these targets, from all the radiations, are due to single independent tracks; the multi-track component is negligibly small. The absolute numbers of energy depositions of approximately greater than 100 eV in DNA segments in a cell are similar to experimentally measured numbers of DNA double-strand breaks, but both these sets of numbers are one or two orders of magnitude larger than the numbers of lethal events produced in mammalian cells. The frequency of threshold energy of approximately 120 eV in a DNA segment correlates reasonably well with the relative biological effectiveness of ultrasoft X-rays and low-LET radiations for relatively radioresistant cells, but a lower threshold energy may be required for other, more sensitive, cells.