a GSI Helmholtzzentrum für Schwerionenforschung, Department of Biophysics, Darmstadt, Germany.
b Technische Universität Darmstadt, Institut für Festkörperphysik, Darmstadt, Germany.
Radiat Res. 2018 Jan;189(1):32-43. doi: 10.1667/RR14653.1. Epub 2017 Oct 26.
There is experimental evidence that ultrasoft X rays (0.1-5 keV) show a higher biological effectiveness than high-energy photons. Similar to high-LET radiation, this is attributed to a rather localized dose distribution associated with a considerably smaller range of secondary electrons, which results in an increasing yield of double-strand breaks (DSBs) and potentially more complex lesions. We previously reported on the application of the Giant LOop Binary LEsion (GLOBLE) model to ultrasoft X rays, in which experimental values of the relative biological effectiveness (RBE) for DSB induction were used to show that this increasing DSB yield was sufficient to explain the enhanced effectiveness in the cell inactivation potential of ultrasoft X rays. Complementary to GLOBLE, we report here on a modeling approach to predict the increased DSB yield of ultrasoft X rays on the basis of amorphous track structure formed by secondary electrons, which was derived from Monte Carlo track structure simulations. This procedure is associated with increased production of single-strand break (SSB) clusters, which are caused by the highly localized energy deposition pattern induced by low-energy photons. From this, the RBE of ultrasoft X rays can be determined and compared to experimental data, showing that the inhomogeneity of the energy deposition pattern represents the key variable to describe the increased biological effectiveness of ultrasoft X rays. Thus, this work demonstrates an extended applicability of the amorphous track structure concept and tests its limits with respect to its predictive power. The employed model mechanism offers a possible explanation for how the cellular response to ultrasoft X rays is directly linked to the energy deposition properties on the nanometric scale.
有实验证据表明，超软 X 射线（0.1-5keV）比高能光子具有更高的生物效应。与高 LET 辐射类似，这归因于与二次电子的小得多的射程相关的相当局部的剂量分布，这导致双链断裂（DSB）的产量增加，并且潜在地更复杂的损伤。我们之前曾报道过将巨环二进制损伤（GLOBLE）模型应用于超软 X 射线，其中使用了 DSB 诱导的相对生物效应（RBE）的实验值来表明，这种增加的 DSB 产量足以解释超软 X 射线在细胞失活潜力方面的增强效果。作为 GLOBLE 的补充，我们在此报告了一种基于由二次电子形成的无定形轨迹结构来预测超软 X 射线增加的 DSB 产量的建模方法，该方法源自蒙特卡罗轨迹结构模拟。该程序与单链断裂（SSB）簇的产生增加有关，这是由低能光子诱导的高度局部能量沉积模式引起的。由此，可以确定超软 X 射线的 RBE，并将其与实验数据进行比较，表明能量沉积模式的非均质性是描述超软 X 射线增加的生物效应的关键变量。因此，这项工作证明了无定形轨迹结构概念的扩展适用性，并测试了其在预测能力方面的局限性。所采用的模型机制为超软 X 射线的细胞反应如何直接与纳米尺度上的能量沉积特性相关提供了一种可能的解释。
