Hill M A, Vecchia M D, Townsend K M, Goodhead D T
Medical Research Council, Radiation and Genome Stability Unit, Didcot, Oxon, UK.
Phys Med Biol. 1998 Feb;43(2):351-63. doi: 10.1088/0031-9155/43/2/010.
Ultrasoft x-rays provide a unique tool for investigating the intracellular mechanisms of radiation action. Secondary electrons are produced with a well defined energy and a range comparable with that of critical structures in the cell. Copper L characteristic x-rays of weighted average energy of 956 eV interact within the cell, mainly with the oxygen atom, typically producing a photoelectron with energy 424 eV (95%) followed by an Auger electron with an average energy of 505 eV, with a combined continuous slowing down approximation (csda) range of approximately 40 nm. The attenuation through the cell is similar to that of carbon K x-rays (277 eV, single electron), therefore a useful comparison can be made due to similar dose-averaging factors but different electron configurations (total range, and pairs versus singlets). The production, absorption, dosimetry and biological implications of Cu L x-rays using the Medical Research Council cold cathode source is described extending the number of energies available for study in the ultrasoft region. Design parameters were optimized to overcome the inherently low L-characteristic-to-bremsstrahlung yield ratio. Surface absorbed dose rates of 1 Gy min-1 have been obtained with a bremsstrahlung contamination of less than 0.5%. A confocal microscope was used to make thickness measurements on live cells to allow careful determination of the mean absorbed dose. Survival curves for V79-4 Chinese hamster cells were obtained, showing that Cu L x-rays are substantially more lethal per unit dose than are hard x-rays or gamma-rays, with a relative biological effectiveness (RBE) of 1.8. The data are consistent with the hypothesis that clustered damage at the DNA/chromatin level produced by low-energy electrons is biologically more effective.
超软X射线为研究辐射作用的细胞内机制提供了一种独特的工具。二次电子产生时具有明确的能量，其射程与细胞内关键结构的射程相当。加权平均能量为956 eV的铜L特征X射线在细胞内相互作用，主要与氧原子相互作用，通常产生能量为424 eV的光电子（95%），随后是平均能量为505 eV的俄歇电子，其组合连续慢化近似（csda）射程约为40 nm。穿过细胞的衰减与碳K X射线（277 eV，单电子）的衰减相似，因此，由于剂量平均因子相似但电子构型不同（总射程以及成对与单态），可以进行有用的比较。本文描述了使用医学研究委员会冷阴极源产生、吸收、剂量测定及铜L X射线的生物学意义，扩展了超软区域可用于研究的能量数量。优化了设计参数以克服固有的低L特征与轫致辐射产额比。已获得表面吸收剂量率为1 Gy min-1且轫致辐射污染小于0.5%的结果。使用共聚焦显微镜对活细胞进行厚度测量，以便仔细确定平均吸收剂量。获得了V79-4中国仓鼠细胞的存活曲线，表明铜L X射线每单位剂量的致死性比硬X射线或γ射线高得多，相对生物效能（RBE）为1.8。这些数据与低能电子在DNA/染色质水平产生的簇状损伤在生物学上更有效的假设一致。
