Goodhead D T, Thacker J, Cox R
Phys Med Biol. 1981 Nov;26(6):1115-27. doi: 10.1088/0031-9155/26/6/010.
This paper tests whether photon absorption processes in particular atomic element(s) may be responsible for the observed high relative biological effectiveness (RBE) of ultrasoft X-rays. The effectiveness of titanium K characteristic X-rays (4.55 keV) is compared with previous observations for aluminium (1.5 keV) and carbon (0.28 keV) K ultrasoft X-rays. For a given absorbed dose, five times more Ti K than Al K photons are absorbed in phosphorus; since Al K X-rays are observed to be more effective in killing human and hamster cells it is concluded that absorption in phosphorus does not play a dominant lethal role. This is supported by the observation that the absolute number of Al K photons absorbed in phosphorus of DNA of human fibroblasts is less than 1 per lethal event. For no element is the relative number of absorbed photons of the three X-ray energies even approximately proportional to their observed RBEs. The effectiveness of ultrasoft X-rays is apparently not due to selective absorption but rather to the secondary electrons; consequently the mechanism of action should be common to the large numbers of low energy secondary electrons produced by most other ionising radiations, including gamma-rays.
本文测试了特定原子元素中的光子吸收过程是否可能是观察到的极软X射线的高相对生物效应（RBE）的原因。将钛K特征X射线（4.55 keV）的有效性与先前对铝（1.5 keV）和碳（0.28 keV）K极软X射线的观察结果进行了比较。对于给定的吸收剂量，在磷中吸收的Ti K光子比Al K光子多五倍；由于观察到Al K X射线在杀死人类和仓鼠细胞方面更有效，因此得出结论，在磷中的吸收并不起主要的致死作用。这一结论得到了以下观察结果的支持：在人类成纤维细胞DNA的磷中，每致死事件吸收的Al K光子绝对数量少于1个。对于任何元素，三种X射线能量的吸收光子相对数量甚至都与它们观察到的RBEs大致不成比例。极软X射线的有效性显然不是由于选择性吸收，而是由于二次电子；因此，作用机制应该与大多数其他电离辐射（包括γ射线）产生的大量低能二次电子相同。
