Lee Sung-Woo, Reece Warren Daniel
Department of Nuclear Engineering, Texas A&M University, College Station, TX 77843-3133, USA.
Phys Med Biol. 2004 Feb 21;49(4):583-99. doi: 10.1088/0031-9155/49/4/009.
Electron backscattering is a prominent secondary effect in beta dosimetry, and is closely correlated with factors such as the geometries of the source and the scattering material, as well as the composition of the scattering material. Previous results were varied depending on the experimental setup and detector resolution, and were generally performed for monoenergetic electron beams, which makes direct application of these factors to beta sources difficult. In this work, backscattering factors were calculated with MCNP 4C for selected beta sources currently in use (32P and 90Sr/90Y) as well as for sources of potential therapeutic use (45Ca, 142Pr and 185W). Specifically, the calculations used beta spectra generated by the SADDE MOD2 code. The factors were calculated as a function of the distance from the interface between water and scatterers. The scatterers include source surroundings, source supporting materials and contrast agents commonly used for imaging purposes in brachytherapy. The results were fit by a simple function for future incorporation into a dose point kernel code. Due to the high-Z material content (iodine, Z = 53) in the contrast agent, a significant dose backscatter was observed near the water interface. Different cross-section algorithms in the MCNP code (inherent and ITS 3.0) affect the factor calculations. The results generated by the ITS 3.0 algorithm closely matched the EGS4 calculations for 32P. The dependence of backscatter factors on log(Z + 1) (Baily 1980 Med. Phys. 7514-9) was observed for all the beta sources with a high correlation coefficient, R (> 0.95). The overall results indicate that the backscattering effect is significant at short distances from the surface of the interface between water and the scattering material. This model can also aid in choosing a source support or mixing materials for beta brachytherapy sources.
电子背散射是β剂量测定中的一个显著二次效应，并且与源和散射材料的几何形状以及散射材料的组成等因素密切相关。先前的结果因实验装置和探测器分辨率而异，并且通常是针对单能电子束进行的，这使得将这些因素直接应用于β源变得困难。在这项工作中，使用MCNP 4C计算了当前使用的选定β源（32P和90Sr/90Y）以及潜在治疗用途的源（45Ca、142Pr和185W）的背散射因子。具体而言，计算使用了由SADDE MOD2代码生成的β能谱。这些因子是作为距水与散射体之间界面的距离的函数来计算的。散射体包括源周围环境、源支撑材料以及近距离放射治疗中常用于成像目的的造影剂。结果通过一个简单函数进行拟合，以便将来纳入剂量点核代码。由于造影剂中高Z材料含量（碘，Z = 53），在水界面附近观察到显著的剂量背散射。MCNP代码中的不同截面算法（固有算法和ITS 3.0）会影响因子计算。ITS 3.0算法生成的结果与32P的EGS4计算结果紧密匹配。对于所有β源，均观察到背散射因子对log(Z + 1)的依赖性（Baily 1980年，《医学物理》7 514 - 9），相关系数R很高（> 0.95）。总体结果表明，在距水与散射材料之间界面表面较短距离处，背散射效应很显著。该模型还可有助于为β近距离放射治疗源选择源支撑材料或混合材料。
