University of Namur, PMR, 61 rue de Bruxelles, 5000 Namur, Belgium.
Phys Med Biol. 2019 Mar 13;64(6):065014. doi: 10.1088/1361-6560/ab0516.
In protontherapy, it has been suggested that nanoparticles of high-Z material like gold (GNP) could be used as radiosensitizers. The origin of this enhancement phenomenon for proton radiation is not yet well understood and additional mechanistic insights are required. Previous works have highlighted the good capabilities of TRAX to reproduce secondary electron emission from gold material. Therefore, TRAX cross sections obtained with the binary encounter approximation (BEA) model for proton ionization were implemented within Geant4 for gold material. Based on the TRAX cross sections, improved Geant4 simulations have been developed to investigate the energy deposition and radical species production around a spherical gold nanoparticle (5 and 10 nm in diameter) placed in a water volume during proton irradiation. Simulations were performed for incident 2 MeV proton. The dose enhancement factor and the radiolysis enhancement factor were quantified. Results obtained with the BEA model were compared with results obtained with condensed-history models. Experimental irradiation of 200 nm gold films were performed to validate the secondary electron emission reproduction capabilities of physical models used in Monte Carlo (MC) simulations. TRAX simulations reproduced the experimental backscattered electron energy spectrum from gold film with better agreement than Geant4. Results on gold film obtained with the BEA model enabled to estimate the electron emission from GNPs. Results obtained in our study tend to support that the use of the BEA discrete model leads to a significant increase of the dose in the near vicinity of GNPs (<20 nm), while condensed history models used in Geant4 seem to overestimate the dose and the number of chemical species for increasing distances from the GNP. Based on discrete BEA model results, no enhancement effect due to secondary electron emitted from the GNP is expected if the GNP is not in close proximity to key cellular functional elements (DNA, mitochondria…).
在质子治疗中，有人提出可以使用高 Z 材料（如金）的纳米颗粒作为放射增敏剂。质子辐射增强现象的起源尚不清楚，需要更多的机制见解。先前的工作已经强调了 TRAX 重现金材料二次电子发射的良好能力。因此，在 Geant4 中实现了用于质子电离的二进制碰撞近似（BEA）模型的 TRAX 截面。基于 TRAX 截面，开发了改进的 Geant4 模拟，以研究在质子辐照期间放置在水中的 5nm 和 10nm 直径的球形金纳米颗粒周围的能量沉积和自由基产生。模拟了 2MeV 质子的入射。量化了剂量增强因子和辐解增强因子。BEA 模型的结果与凝聚历史模型的结果进行了比较。对 200nm 金膜进行了实验辐照，以验证蒙特卡罗（MC）模拟中使用的物理模型的二次电子发射再现能力。TRAX 模拟比 Geant4 更好地再现了金膜的实验背散射电子能谱。BEA 模型在金膜上的结果使我们能够估计 GNPs 的电子发射。我们的研究结果表明，使用 BEA 离散模型会导致在 GNPs 附近（<20nm）剂量显著增加，而 Geant4 中使用的凝聚历史模型似乎会高估剂量和化学物质的数量随着与 GNP 的距离增加。基于离散 BEA 模型的结果，如果 GNP 不接近关键的细胞功能元件（DNA、线粒体……），则不会由于从 GNP 发射的二次电子而产生增强效应。
