Australian Nuclear Science and Technology Organisation (ANSTO), Lucas Heights, NSW, Australia.
Int J Radiat Biol. 2012 Jan;88(1-2):164-70. doi: 10.3109/09553002.2011.627975.
Lethal cell damage by ionising radiation is generally initiated by the formation of complex strand breaks, resulting from ionisation clusters in the DNA molecule. A better understanding of the effect of the distribution of ionisation clusters within the cell and particularly in regard to DNA segments could be beneficial to radiation therapy treatment planning. Low energy X-rays generate an abundance of low energy electrons similar to that associated with MeV protons. The study and comparison of the track structure of photon and proton beams could permit the substitution of photon microbeams for single cell ion irradiations at proton facilities used to predict the relative biological effectiveness (RBE) of charged particle fields.
The track structure of X-ray photons is compared with proton pencil beams in voxels of approximate DNA strand size (2 × 2 × 5 nm). The Very Low Energy extension models of the Monte Carlo simulation toolkit GEometry ANd Tracking 4 (Geant4) is used. Simulations were performed in a water phantom for an X-ray and proton beam of energies 100 keV and 20 MeV, respectively.
The track structure of the photon and proton beams are evaluated using the ionisation cluster size distribution as well as the radial dose deposition of the beam.
A comparative analysis of the ionisation cluster distribution and radial dose deposition obtained is presented, which suggest that low energy X-rays could produce similar ionisation cluster distributions to MeV protons on the DNA scale of size at depths greater than ∼10 μm and at distances greater than ∼1 μm from the beam centre. Here the ionisation cluster size for each beam is less than ∼100. The radial dose deposition is also approximately equal at large depths and at distances greater than 10 μm from the beam centre.
电离辐射导致的致死性细胞损伤通常是由 DNA 分子中离子簇形成的复杂链断裂引起的。更好地了解细胞内离子簇的分布情况，特别是 DNA 片段的分布情况，可能有助于放射治疗计划的制定。低能 X 射线会产生大量类似于与 MeV 质子相关的低能电子。光子和质子束的轨迹结构的研究和比较可以允许用光子微束代替质子设施中的单细胞离子辐照，以预测带电粒子场的相对生物效应（RBE）。
将 X 射线光子的轨迹结构与质子铅笔束在近似 DNA 链大小的体素（2×2×5nm）中进行比较。使用蒙特卡罗模拟工具包 GEometry ANd Tracking 4（Geant4）的非常低能扩展模型。在水模体中对能量分别为 100keV 和 20MeV 的 X 射线和质子束进行了模拟。
使用电离簇大小分布以及束的径向剂量沉积来评估光子和质子束的轨迹结构。
对获得的电离簇分布和径向剂量沉积进行了比较分析，结果表明，在大于约 10μm 的深度和大于约 1μm 的距离处，低能 X 射线在 DNA 大小的范围内可能会产生类似于 MeV 质子的类似电离簇分布。在每个束中，离子簇的大小都小于约 100。在大深度和距离大于束中心 10μm 处，径向剂量沉积也大致相等。
