a Medical Radiation Physics, Department of Immunology, Genetics and Pathology, Uppsala University, Akademiska Sjukhuset, Uppsala SE-75185, Sweden.
b Elekta Instrument AB, Stockholm SE-10393, Sweden.
Radiat Res. 2018 Nov;190(5):504-512. doi: 10.1667/RR15089.1. Epub 2018 Aug 14.
The linear-quadratic (LQ) parameterization of survival fraction [SF( D)] inherently assumes that all cells in a population receive the same dose ( D), albeit the distribution of specific energy z over the individual cells f( z, D) can be very wide. From these microdosimetric distributions, which are target size dependent, we estimate the size of the cellular sensitive volume by analyzing its influence on the LQ parameterization of cell survival. A Monte Carlo track structure code was used to simulate detailed tracks from a Co source as well as proton and carbon ions of various energies. From these tracks, f( z, D) distributions were calculated for spherical targets with diameters ranging from 10 nm to 12 μm. A cell survival function based on f( z, D) was fitted to experimental LQ α values, revealing an intrinsic limitation that target size imposes on the usage of f( z, D) to describe the linear term of the LQ parameterization. The results indicate that such threshold volume arises naturally from the relationship between the particle's probability of no-hit and the probability of cell survival. Further analysis led to the proposal of a radiobiological property [Formula: see text], defined as the mean lineal energy corresponding to the target size that allows equivalence between the mean inactivation dose (MID) and the mean specific energy [Formula: see text]. The fact that [Formula: see text] is an increasing continuous function of target size within the range of biological targets of interest in radiobiology, ensures the uniqueness of [Formula: see text] for any radiation quality, thus, its potential usefulness in modeling. In conclusion, an accurate estimation of such threshold volumes may be useful for improving modeling of cell survival curves.
生存分数 SF(D) 的线性二次 (LQ) 参数化假设群体中的所有细胞都接受相同的剂量 D，尽管个体细胞中特定能量 z 的分布 f(z, D) 可能非常广泛。从这些与靶大小相关的微剂量分布中，我们通过分析其对细胞存活 LQ 参数化的影响来估计细胞敏感体积的大小。使用蒙特卡罗轨道结构代码模拟了来自 Co 源以及各种能量的质子和碳离子的详细轨迹。从这些轨迹中，为直径范围从 10nm 到 12μm 的球形靶计算了 f(z, D) 分布。基于 f(z, D) 的细胞存活函数与实验 LQ α 值拟合，揭示了靶大小对使用 f(z, D) 来描述 LQ 参数化的线性项的固有限制。结果表明，这种阈值体积是由于粒子无命中概率与细胞存活概率之间的关系自然产生的。进一步的分析导致提出了一种放射生物学性质 [公式：见文本]，定义为允许平均失活剂量 (MID) 与平均特定能量 [公式：见文本] 之间等效的靶大小对应的平均线性能量。在放射生物学中感兴趣的生物靶大小范围内，[公式：见文本] 是靶大小的连续递增函数，这确保了对于任何辐射质量，[公式：见文本] 的唯一性，因此，它在建模中的潜在有用性。总之，这种阈值体积的准确估计可能有助于改进细胞存活曲线的建模。
