Nelson Gregory A
Loma Linda University, Loma Linda, CA, USA.
Gravit Space Biol Bull. 2003 Jun;16(2):29-36.
The unique feature of the space radiation environment is the dominance of high-energy charged particles (HZE or high LET radiation) emitted by the Sun and galactic sources, or trapped in the Van Allen radiation belts. These charged particles present a significant hazard to space flight crews, and accelerator-based experiments are underway to quantify the health risks due to unavoidable radiation exposure. There are three principal properties of charged particles that distinguish them from conventional radiation, i.e. gamma rays and x-rays. First, they have a defined range in matter rather than an exponential absorption profile. Second, they undergo nuclear reactions to produce secondary particles. Third, and most important, they deposit their energy along well-defined linear paths or tracks rather than diffuse fields. The structured energy deposition pattern interacts on multiple scales with the biological structures of DNA, cells and tissues to produce correlated patterns of damage that evade repair systems. Traditional concepts of dose and its associated normalization parameter, RBE (relative biological effectiveness), break down under experimental scrutiny, and probabilistic models of risk based on the number of particle traversals per cell may be more appropriate. Unique patterns of DNA damage, gene expression, mobilization of repair proteins, activation of cytokines and remodeling of cellular microenvironment are observed following exposure to high LET radiation. At low levels of exposure the communication of bioactive substances from irradiated to unirradiated "bystander" cells can amplify the damage and cause a significant deviation from linearity in dose vs. response relations. Under some circumstances, there is even a multigenerational delay in the expression of radiation-induced genetic damage (genomic instability) which is not strictly dose dependent. These issues and the experimental evidence derived from ground based experiments at particle accelerators are presented along with speculation about how modified inertial conditions might perturb homeostatic responses to radiation to further complicate risk assessment for space flight.
空间辐射环境的独特之处在于，来自太阳和星系源或被困在范艾伦辐射带中的高能带电粒子（HZE或高传能线密度辐射）占主导地位。这些带电粒子对航天机组人员构成重大危害，目前正在进行基于加速器的实验，以量化不可避免的辐射暴露所带来的健康风险。带电粒子有三个主要特性，使其有别于传统辐射，即伽马射线和X射线。其一，它们在物质中有确定的射程，而非指数吸收分布。其二，它们会发生核反应以产生次级粒子。其三，也是最重要的一点，它们沿明确的线性路径或轨迹沉积能量，而非扩散场。这种结构化的能量沉积模式在多个尺度上与DNA、细胞和组织的生物结构相互作用，产生逃避修复系统的相关损伤模式。传统的剂量概念及其相关的归一化参数RBE（相对生物效能）在实验检验下站不住脚，基于每个细胞粒子穿越次数的概率风险模型可能更为合适。在暴露于高传能线密度辐射后，可观察到独特的DNA损伤、基因表达、修复蛋白动员、细胞因子激活和细胞微环境重塑模式。在低暴露水平下，生物活性物质从受辐照细胞传递到未受辐照的“旁观者”细胞，可放大损伤，并导致剂量与反应关系显著偏离线性。在某些情况下，甚至存在辐射诱导的遗传损伤（基因组不稳定）表达的多代延迟，这并非严格依赖剂量。本文介绍了这些问题以及在粒子加速器上进行的地面实验所获得的实验证据，并推测了改变的惯性条件可能如何扰乱对辐射的稳态反应，从而使太空飞行的风险评估更加复杂。
