Laboratório de Cidncias Radioldgicas, Universidade do Estado do Rio de Janeiro, Rio de Janeiro 20550-90, Brazil.
Med Phys. 2011 Jul;38(7):4147-53. doi: 10.1118/1.3597568.
The invariance of the total direct strand break yield when DNA is irradiated by different types of particles and energies has been reported by previous works. This study is intended to explain the physical causes of this behavior.
The GEANT4-DNA extension of the GEANT4 general purpose Monte Carlo simulation toolkit has been used to determine direct strand break yields induced by protons and alpha particles impacting on a B-DNA geometrical model, including five organization levels of the human genetic material. The linear energy transfer (LET) of such particles ranges from 4.8 keV/microm (10 MeV protons) to about 235 keV/microm (2 MeV alpha particles), at 5.225 pm depth (near the center of the region of interest). Direct total, single and double strand break probabilities have been determined in a liquid water homogeneous medium with a 1.06 g/cm3 density. The energetic spectra of single strand breaks (SSB), the number of energy deposition events, and the SSB/event ratio were determined.
The target-hit probability was found to be independent of both the type and the energy of the incident particle, even if this latter is a secondary electron. This probability is determined by the geometrical properties of the system. The total strand break yield and the number of energy deposition events required to reach a certain absorbed dose were found nearly independent of the type and energy of the incident ion (proton or alpha). In contrast, the double strand break (DSB) yield was found strongly dependent on the LET of the incident radiation.
The SSB generation process is homogeneous and independent of the LET of the particles involved, at least within the proton and alpha particle energy range here studied. The target-hit probability is only determined by the ratio between the total volume occupied by targets and that of the ROI where the radiation deposits its energy. The maximum separation distance between two adjacent SSBs to produce a DSB is the parameter that breaks the homogeneity of the target-hit process, making the DSB production process strongly heterogeneous.
先前的工作已经报道了不同类型的粒子和能量照射 DNA 时总直接链断裂产额的不变性。本研究旨在解释这种行为的物理原因。
使用 GEANT4 通用蒙特卡罗模拟工具包的 GEANT4-DNA 扩展，确定质子和α粒子撞击 B-DNA 几何模型时产生的直接链断裂产额，包括人类遗传物质的五个组织层次。这些粒子的线性能量转移（LET）范围从 4.8 keV/μm（10 MeV 质子）到约 235 keV/μm（2 MeV α粒子），在 5.225 pm 深度（感兴趣区域的中心附近）。在密度为 1.06 g/cm3 的液体水中，确定了直接总、单链和双链断裂的概率。确定了单链断裂（SSB）的能量谱、能量沉积事件的数量以及 SSB/事件比。
发现目标命中概率与入射粒子的类型和能量都无关，即使后者是次级电子。这种概率由系统的几何性质决定。总链断裂产额和达到一定吸收剂量所需的能量沉积事件数量几乎与入射离子（质子或α）的类型和能量无关。相比之下，双链断裂（DSB）产额与入射辐射的 LET 强烈相关。
SSB 产生过程是均匀的，与所涉及粒子的 LET 无关，至少在所研究的质子和α粒子能量范围内是如此。目标命中概率仅由目标总体积与辐射沉积能量的 ROI 体积之比决定。产生 DSB 的两个相邻 SSB 之间的最大分离距离是打破目标命中过程均匀性的参数，使 DSB 产生过程具有强烈的异质性。
