Spiga J, Siegbahn E A, Bräuer-Krisch E, Randaccio P, Bravin A
European Synchrotron Radiation Facility (ESRF), B.P.220, 6, rue Horowitz, 38043 Grenoble Cedex, France.
Med Phys. 2007 Nov;34(11):4322-30. doi: 10.1118/1.2794170.
Theoretical dose distributions for microbeam radiation therapy (MRT) are computed in this paper using the GEANT4 Monte Carlo (MC) simulation toolkit. MRT is an innovative experimental radiotherapy technique carried out using an array of parallel microbeams of synchrotron-wiggler-generated x rays. Although the biological mechanisms underlying the effects of microbeams are still largely unknown, the effectiveness of MRT can be traced back to the natural ability of normal tissues to rapidly repair small damages to the vasculature, and on the lack of a similar healing process in tumoral tissues. Contrary to conventional therapy, in which each beam is at least several millimeters wide, the narrowness of the microbeams allows a rapid regeneration of the blood vessels along the beams' trajectories. For this reason the calculation of the "valley" dose is of crucial importance and the correct use of MC codes for such purposes must be understood. GEANT4 offers, in addition to the standard libraries, a specialized package specifically designed to deal with electromagnetic interactions of particles with matter for energies down to 250 eV. This package implements two different approaches for electron and photon transport, one based on evaluated data libraries, the other adopting analytical models. These features are exploited to cross-check theoretical computations for MRT. The lateral and depth dose profiles are studied for the irradiation of a 20 cm diameter, 20 cm long cylindrical phantom, with cylindrical sources of different size and energy. Microbeam arrays are simulated with the aid of superposition algorithms, and the ratios of peak-to-valley doses are computed for typical cases used in preclinical assays. Dose profiles obtained using the GEANT4 evaluated data libraries and analytical models are compared with simulation results previously obtained using the PENELOPE code. The results show that dose profiles computed with GEANT4's analytical model are almost indistinguishable from those obtained with the PENELOPE code, but some noticeable differences appear when the evaluated data libraries are used.
本文使用GEANT4蒙特卡罗（MC）模拟工具包计算了微束放射治疗（MRT）的理论剂量分布。MRT是一种创新的实验性放射治疗技术，利用同步加速器摆动器产生的X射线平行微束阵列进行。尽管微束效应背后的生物学机制在很大程度上仍然未知，但MRT的有效性可追溯到正常组织快速修复血管小损伤的天然能力，以及肿瘤组织中缺乏类似的愈合过程。与传统疗法不同，传统疗法中每个光束至少几毫米宽，微束的狭窄允许血管沿光束轨迹快速再生。因此，“谷”剂量的计算至关重要，必须理解为此目的正确使用MC代码。除了标准库之外，GEANT4还提供了一个专门的包，专门设计用于处理能量低至250 eV的粒子与物质的电磁相互作用。该包实现了两种不同的电子和光子传输方法，一种基于评估数据库，另一种采用解析模型。利用这些特性对MRT的理论计算进行交叉检验。研究了用不同尺寸和能量的圆柱形源照射直径20 cm、长20 cm的圆柱形体模时的横向和深度剂量分布。借助叠加算法模拟微束阵列，并计算临床前试验中典型案例的峰谷剂量比。将使用GEANT4评估数据库和解析模型获得的剂量分布与先前使用PENELOPE代码获得的模拟结果进行比较。结果表明，用GEANT4解析模型计算的剂量分布与用PENELOPE代码获得的剂量分布几乎无法区分，但使用评估数据库时会出现一些明显差异。
