Ahmad Syed Bilal, Sarfehnia Arman, Kim Anthony, Wronski Matt, Sahgal Arjun, Keller Brian M
Sunnybrook Research Institute, 2075 Bayview Avenue, Toronto, Ontario M4N 3M5, Canada and Sunnybrook Health Sciences Center, Odette Cancer Center, 2075 Bayview Avenue, Toronto, Ontario M4N 3M5, Canada.
Sunnybrook Health Sciences Center, Odette Cancer Center, 2075 Bayview Avenue, Toronto, Ontario M4N 3M5, Canada and Department of Radiation Oncology, University of Toronto, 27 King's College Circle, Toronto, Ontario M5S 1A1, Canada.
Med Phys. 2016 Aug;43(8):4665. doi: 10.1118/1.4955175.
To quantify and explain the backscatter dose effects for clinically relevant high atomic number materials being irradiated in the presence of a 1.5 T transverse magnetic field.
Interface effects were investigated using Monte Carlo simulation techniques. We used gpumcd (v5.1) and geant4 (v10.1) for this purpose. gpumcd is a commercial software written for the Elekta AB, MRI linac. Dose was scored using gpumcd in cubic voxels of side 1 and 0.5 mm, in two different virtual phantoms of dimensions 20 × 20 × 20 cm and 5 × 5 × 13.3 cm, respectively. A photon beam was generated from a point 143.5 cm away from the isocenter with energy distribution sampled from a histogram representing the true Elekta, MRI linac photon spectrum. A slab of variable thickness and position containing either bone, aluminum, titanium, stainless steel, or one of the two different dental filling materials was inserted as an inhomogeneity in the 20 × 20 × 20 cm phantom. The 5 × 5 × 13.3 cm phantom was used as a clinical test case in order to explain the dose perturbation effects for a head and neck cancer patient. The back scatter dose factor (BSDF) was defined as the ratio of the doses at a given depth with and without the presence of the inhomogeneity. Backscattered electron fluence was calculated at the inhomogeneity interface using geant4. A 1.5 T magnetic field was applied perpendicular to the direction of the beam in both phantoms, identical to the geometry in the Elekta MRI linac.
With the application of a 1.5 T magnetic field, all the BSDF's were reduced by 12%-47%, compared to the no magnetic field case. The corresponding backscattered electron fluence at the interface was also reduced by 45%-64%. The reduction in the BSDF at the interface, due to the application of the magnetic field, is manifested in a different manner for each material. In the case of bone, the dose drops at the interface contrary to the expected increase when no magnetic field is applied. In the case of aluminum, the dose at the interface is the same with and without the presence of the aluminum. For all of the other materials the dose increases at the interface.
The reduction in dose at the interface, in the presence of the magnetic field, is directly related to the reduction in backscattered electron fluence. This reduction occurs due to two different reasons. First, the electron spectrum hitting the interface is changed when the magnetic field is turned on, which results in changes in the electron scattering probability. Second, some electrons that have curved trajectories due to the presence of the magnetic field are absorbed by the higher density side of the interface and no longer contribute to the backscattered electron fluence.
量化并解释在1.5 T横向磁场存在的情况下，临床相关高原子序数材料受照射时的反向散射剂量效应。
使用蒙特卡罗模拟技术研究界面效应。为此我们使用了gpumcd（v5.1）和geant4（v10.1）。gpumcd是一款为医科达公司的MRI直线加速器编写的商业软件。在边长为1和0.5 mm的立方体素中，分别在尺寸为20×20×20 cm和5×5×13.3 cm的两个不同虚拟体模中，使用gpumcd对剂量进行评分。从距离等中心143.5 cm处的一个点产生光子束，其能量分布从代表医科达MRI直线加速器真实光子能谱的直方图中采样。将一块厚度和位置可变、包含骨、铝、钛、不锈钢或两种不同牙科填充材料之一的平板作为不均匀性插入到20×20×20 cm的体模中。5×5×13.3 cm的体模用作临床测试案例，以解释头颈癌患者的剂量扰动效应。反向散射剂量因子（BSDF）定义为给定深度处有和没有不均匀性时的剂量之比。使用geant4在不均匀性界面处计算反向散射电子注量。在两个体模中，均施加一个垂直于束流方向的1.5 T磁场，其几何结构与医科达MRI直线加速器中的相同。
与无磁场情况相比，施加1.5 T磁场后，所有的BSDF均降低了12% - 47%。界面处相应的反向散射电子注量也降低了45% - 64%。由于施加磁场，界面处BSDF的降低对每种材料的表现方式不同。对于骨，界面处的剂量下降，这与无磁场时预期的增加相反。对于铝，有无铝存在时界面处的剂量相同。对于所有其他材料，界面处的剂量增加。
在有磁场的情况下，界面处剂量的降低与反向散射电子注量的降低直接相关。这种降低是由两个不同原因导致的。首先，开启磁场时，撞击界面的电子能谱发生变化，这导致电子散射概率发生变化。其次，一些由于磁场存在而具有弯曲轨迹的电子被界面的高密度侧吸收，不再对反向散射电子注量有贡献。
