高Z纳米介质放射增敏的宏观剂量效应评价及其局限

Macroscopic Dosimetric Evaluiation and Its Limitations for High-Z Nano Particle Radiosensitizer

目的:建立高Z纳米介质对X-γ射线和质子/重离子的宏观剂量增强效应模型,讨论其评价高Z纳米介质放射增敏效应的局限性。方法:基于宏观剂量学的质能吸收系数、质量阻止本领、电子阻止本领以及传能线密度(LET)概念,推导组织内均匀分布的高Z纳米介质对X-γ光子、质子或重离子的宏观剂量增强比;分析剂量增强比随射线能量、纳米介质原子序数和组织内含量的变化规律;对比放射增敏比实验研究结果,讨论宏观剂量学模型评价高Z纳米介质放射增敏效应的局限性。结果:宏观剂量学分析表明高Z纳米介质可以对X-γ射线和质子/重离子产生宏观剂量增强效应,但基于多重均匀化假设的宏观剂量增强比计算结果对实际放射增敏效应有明显低估。结论:高Z纳米介质放射增敏的准确量效关系需建立严格的微观剂量学模型评价,微观剂量学模型需要考虑四方面因素:(1)次级电子局域剂量分布;(2)纳米颗粒非均匀团聚分布;(3)纳米颗粒亚细胞分布;(4)局域剂量的自由基效应。

Objetcive： Developing a macroscopic dosimetry model for the dose enhancement effect ofhigh-Z nanopartieles on photon, proton and heavy ion radiations, and disscussing its limitations in evaluating the radiosensitization effects of high-Z nano particles. Methods： dose enhancement ratio of photons, protons and heavy ions by evenly distributed high-Z nanomate- rials in soft tissues are analytically derived, based on the macroscopic concepts of mass energy absorption coefficient, mass/electronic stopping power and linear energy transfer; the influencing factors like photon/ion energy, atomic number of high-Z material and its tissue content on the dose enhancement ratio are analyzed; the limitations of the macroscopic model in evaluating the dose-response relationship of the radiosensitization effects of high-Z nanoparticles are discussed in comparison with published experimental results, Results： macroscopic dosimetric analysis indicate that high-Z nanomaterials have a dose enhancement effect on ionizing radiations like X-γ ray, proton and heavy ions, but the modeling predictions have an obvious underestimation of the radiosensitization effects of high-Z nanoparticles. Conclusions： For stricter evaluation of the radiosen- sitization effect, mierodosimetry modeling, instead of the homogenization hypotheses in macrosocpic models, is necessary with the essential factors taken into account：（1） local dose deposition of secondary electrons, （2）subcellualr distribution and （3） inhomogeneous agglomeration of high-Z nanoparticles, and （4） free radical effects by the lodcal dose of secondary elec- trons.