Goodhead D T, Nikjoo H
Medical Research Council Radiobiology Unit, Chilton, Didcot, Oxon, U.K.
Int J Radiat Biol. 1989 Apr;55(4):513-29. doi: 10.1080/09553008914550571.
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.
已使用蒙特卡罗径迹结构模拟超软X射线以及选定的低传能线密度和高传能线密度辐射（用于比较），以获得在类似于DNA短片段、核小体和染色质纤维短片段尺寸的小亚细胞靶标中能量沉积的统计有效频率分布。研究发现，在所有情况下，用1 Gy的这些辐射中的任何一种照射单个哺乳动物细胞时，在这些靶标中会发生大量（约10³）直接能量沉积事件。几乎在所有情况下，相当大尺寸的能量沉积数量（例如，DNA片段中约大于100 eV，核小体中约大于300 eV或染色质纤维片段中约大于800 eV）也相当大，每Gy每细胞约为10到100。因此，在评估任何电离辐射对哺乳动物细胞的生物学效应时，显然必须考虑辐射对大分子的直接效应。计算还表明，高传能线密度辐射可在靶标中产生独特的大量能量沉积，这是其他任何辐射几乎无法实现的；这使得高传能线密度辐射有可能造成独特的生化和细胞损伤。对于哺乳动物细胞的任何实际剂量，实际上所有这些辐射在这些靶标中的能量沉积都归因于单个独立径迹；多径迹成分小到可以忽略不计。细胞中DNA片段中约大于100 eV的能量沉积绝对数量与实验测量的DNA双链断裂数量相似，但这两组数量都比哺乳动物细胞中产生的致死事件数量大一个或两个数量级。DNA片段中约120 eV的阈值能量频率与超软X射线和低传能线密度辐射对相对抗辐射细胞的相对生物效能相当好地相关，但其他更敏感的细胞可能需要更低的阈值能量。
