González-Muñoz Gloria, Tilly Nina, Fernández-Varea José M, Ahnesjö Anders
Section of Medical Physics, Department of Oncology, Radiology and Clinical Immunology, Uppsala University, Akademiska Sjukhuset, SE-751 85 Uppsala, Sweden.
Phys Med Biol. 2008 Jun 7;53(11):2857-75. doi: 10.1088/0031-9155/53/11/007. Epub 2008 May 6.
The spatial pattern of energy depositions is crucial for understanding the mechanisms that modify the relative biological effectiveness of different radiation qualities. In this paper, we present data on energy-deposition properties of mono-energetic protons (1-20 MeV) and their secondary electrons in liquid water. Proton-impact ionization was described by means of the Hansen-Kocbach-Stolterfoht doubly differential cross section (DDCS), thus modelling both the initial energy and angle of the emitted electron. Excitation by proton impact was included to account for the contribution of this interaction channel to the electronic stopping power of the projectile. Proton transport was implemented assuming track-segment conditions, whereas electrons were followed down to 50 eV by the Monte Carlo code PENELOPE. Electron intra-track energy-deposition properties, such as slowing-down and energy-imparted spectra of electrons, were calculated. Furthermore, the use of DDCSs enabled the scoring of electron inter-track properties. We present novel results for 1, 5 and 20 MeV single-proton-track frequencies of distances between the nearest inter- (e(-)-e(-), e(-)-H+) and intra-track (e(-)-e(-), e(-)-H+, H+-H+) energy-deposition events. By setting a threshold energy of 17.5 eV, commonly employed as a surrogate to discriminate for elementary damage in the DNA, the variation in these frequencies was studied as well. The energy deposited directly by the proton represents a large amount of the total energy deposited along the track, but when an energy threshold is adopted the relative contribution of the secondary electrons becomes larger for increasing energy of the projectile. We found that the frequencies of closest energy-deposition events per nanometre decrease with proton energy, i.e. for lower proton energies a denser ionization occurs, following the trend of the characteristic LET curves. In conclusion, considering the energy depositions due to the delta electrons and at the core of the track, 1 MeV protons have an intrinsic capability of generating about five times more dual depositions within the characteristic 2 nm of the DNA-chain structure than 20 MeV protons.
能量沉积的空间模式对于理解改变不同辐射品质相对生物有效性的机制至关重要。在本文中，我们展示了单能质子（1 - 20 MeV）及其在液态水中的二次电子的能量沉积特性数据。质子碰撞电离通过汉森 - 科巴赫 - 斯托尔特福赫特双微分截面（DDCS）进行描述，从而对发射电子的初始能量和角度进行建模。考虑了质子碰撞激发，以说明该相互作用通道对射弹电子阻止本领的贡献。质子输运在径迹段条件下实现，而电子则通过蒙特卡罗代码PENELOPE追踪至50 eV。计算了电子径迹内的能量沉积特性，如电子的慢化和能量传递谱。此外，使用DDCS能够对电子径迹间特性进行计分。我们给出了关于1、5和20 MeV单质子径迹中最近的径迹间（e(-)-e(-)，e(-)-H+）和径迹内（e(-)-e(-)，e(-)-H+，H+-H+）能量沉积事件之间距离的频率的新结果。通过设置17.5 eV的阈值能量（通常用作区分DNA中基本损伤的替代值），也研究了这些频率的变化。质子直接沉积的能量占沿径迹沉积的总能量的很大一部分，但当采用能量阈值时，随着射弹能量增加，二次电子的相对贡献会变大。我们发现，每纳米最近能量沉积事件的频率随质子能量降低，即对于较低的质子能量，会发生更密集的电离，遵循特征线性能量传递（LET）曲线的趋势。总之，考虑到δ电子和径迹核心处的能量沉积，1 MeV质子在DNA链结构特征性的2 nm范围内产生双重沉积的内在能力比20 MeV质子大约多五倍。
