High-Z material nanoparticles are being studied as localized dose enhancers in radiotherapeutic applications. Here, the nano-scale physical dose en\-hance\-ment of proton, carbon and oxygen ion beam radiation by gold nanoparticles was studied by means of Monte Carlo track structure simulation with the TRAX code. We present 2D distributions and radial profiles of the additional dose and the dose enhancement factor for two geometries which consider an isolated and a water-em\-bedded nanoparticle, respectively. Different nanoparticle sizes (radius of 1.2-22nm) were found to yield qualitatively different absolute and relative dose enhancement distributions and different maximum dose enhancement factors (up to 20). Whereas the smallest nanoparticles produced the highest local dose enhancement factor close to the metal, larger ones led to lower, more diffuse dose enhancement factors that contributed more at larger distances. Differential absorption effects inside the metal were found to be responsible for those characteristics. For the energy range 15-204 MeV/u, also a mild trend with ion E/A, regardless of the ion species, was found for embedded nanoparticles. In analogy to the width of the ion track itself, slower ions increased the enhancement at the nanoparticle surface. In contrast, no dependence on linear energy transfer was encountered. For slower ions (3-10 MeV/u), the enhancement effect began to break down over all distances. Finally, the significance of any indirect physical effect was excluded, giving important hints especially in view of the low probabilities (at realistic concentrations and fluences) of direct ion-NP-hits. The very localized nature of the physical dose enhancement found suggests a strong action upon targets closeby, but no relevant effect at cellular distances. When pondering different possible damage enhancement mechanisms of gold nanoparticles in the context of published in vitro and in vivo experimental results, biological pathways are likely to play the key role.

中文翻译：

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离子束中金纳米粒子的纳米剂量增强的系统定量。

高Z材料纳米颗粒正在研究作为放射治疗应用中的局部剂量增强剂。在此，利用蒙特卡洛轨道结构模拟和TRAX代码研究了金纳米粒子对质子，碳和氧离子束辐射的纳米级物理剂量增强。我们介绍了两种几何结构的附加剂量和剂量增强因子的2D分布和径向分布，它们分别考虑了一个孤立的和一个嵌入水的纳米颗粒。发现不同的纳米颗粒尺寸（半径为1.2-22nm）在质量上产生绝对和相对的剂量增强分布以及不同的最大剂量增强因子（最多20个）。最小的纳米粒子产生接近金属的最高局部剂量增强因子，而较大的纳米粒子则导致较低的局部剂量增强因子，更多的扩散剂量增加因子，在更大的距离上贡献更大。发现金属内部的吸收差异效应是这些特征的原因。对于15-204 MeV / u的能量范围，对于嵌入的纳米粒子，无论离子种类如何，离子E / A的变化趋势也很小。与离子轨道本身的宽度类似，较慢的离子会增加纳米粒子表面的增强作用。相反，没有遇到对线性能量传递的依赖。对于较慢的离子（3-10 MeV / u），增强作用在所有距离上都开始减弱。最后，排除了任何间接物理影响的重要性，特别是鉴于直接离子NP命中的概率很低（在实际浓度和注量下），因此提供了重要的提示。发现的物理剂量增强的非常局部的性质表明对目标附近有很强的作用，但在细胞距离处没有相关作用。当在已发表的体外和体内实验结果的背景下考虑金纳米颗粒的不同可能的损伤增强机制时，生物学途径可能起关键作用。