Mahdavi S R, Esmaeeli A D, Pouladian M, Monfared A S, Sardari D, Bagheri S
Department of Medical Physics, Iran University of Medical Sciences, Tehran 1449614535, Iran.
Department of Physics, Rasht Branch, Islamic Azad University, Rasht 4147654919, Iran.
Med Phys. 2015 Feb;42(2):925-36. doi: 10.1118/1.4906193.
In the framework of developing the integration of a MRI-Linac system, configurations of MRI-Linac units were simulated in order to improve the dose distribution in tangential breast radiotherapy using transverse and longitudinal magnetic field geometries of Lorentz force for both medial and lateral tangential fields.
In this work, the geant4 Monte Carlo (MC) code was utilized to compare dose distributions in breast radiotherapy for Linac-MR systems in the transverse and longitudinal geometries within humanoid phantoms across a range of magnetic field strengths of 0.5 and 1.5 T. The dose increment due to scattering from the coils was investigated for both geometries as well. Computed tomography images of two patients were used for MC simulations. One patient had intact breast while the other was mastectomized. In the simulations, planning and methods of chest wall irradiation were similar to the actual clinical planning.
In a longitudinal geometry, the magnetic field is shown to restrict the lateral spread of secondary electrons to the lung, heart, and contralateral organs, which reduced the mean dose of the ipsilateral lung and heart by means of 17.2% and 6% at 1.5 T, respectively. The transverse configuration exhibits a significant increase in tissue interface effects, which increased dose buildup in the entrance regions of the lateral and medial tangent beams to the planning target volume (PTV) and improved dose homogeneity within the PTV. The improved relative average homogeneity index for two patients to the PTV at magnetic field strength of 1.5 T with respect to no magnetic field case evaluated was 11.79% and 34.45% in the LRBP and TRBP geometries, respectively. In both geometries, the simulations show significant mean dose reductions in the contralateral breast and chest wall skin, respectively, by a mean of 16.6% and 24.9% at 0.5 T and 17.2% and 28.1% at 1.5 T in the transverse geometry, and 10.56% and 14.6% at 0.5 T and 11.3% and 16.3% at 1.5 T in the longitudinal geometry. Considering the scattered photons which reflected from the coils, the average relative dose of each voxel is slightly increased by 0.53% and 0.32% in the LRBP and TRBP geometries, respectively.
Orienting the B0 magnetic field parallel to the photon beam axis, LRBP geometry, tends to restrict the radial spread of secondary electrons which resulted in dose reduction to the lung. Dosimetry issues observed in both Linac-MR geometries, such as changes to the lateral dose distribution, significantly exhibited dose reduction in the contralateral organs on a representative breast plan. Further, the results show sharper edge dose volume histogram curves at 1.5 T for both geometries, especially in the LRBP configuration.
在开发磁共振直线加速器（MRI-Linac）系统集成的框架下，对MRI-Linac单元的配置进行模拟，以利用洛伦兹力的横向和纵向磁场几何结构改善乳腺切线放疗中的剂量分布，用于内侧和外侧切线野。
在本研究中，利用geant4蒙特卡罗（MC）代码比较了在0.5和1.5 T一系列磁场强度下，类人模型中直线加速器-磁共振（Linac-MR）系统在横向和纵向几何结构中乳腺放疗的剂量分布。还研究了两种几何结构中由于线圈散射导致的剂量增加。使用两名患者的计算机断层扫描图像进行MC模拟。一名患者乳房完整，另一名患者进行了乳房切除。在模拟中，胸壁照射的计划和方法与实际临床计划相似。
在纵向几何结构中，磁场显示出限制二次电子向肺、心脏和对侧器官的横向扩散，这在1.5 T时分别使同侧肺和心脏的平均剂量降低了17.2%和6%。横向配置显示组织界面效应显著增加，这增加了外侧和内侧切线束进入计划靶体积（PTV）区域的剂量积累，并改善了PTV内的剂量均匀性。在1.5 T磁场强度下，相对于无磁场情况，两名患者在LRBP和TRBP几何结构中到PTV的改进相对平均均匀性指数分别为11.79%和34.45%。在两种几何结构中，模拟均显示在横向几何结构中，对侧乳腺和胸壁皮肤的平均剂量分别显著降低，在0.5 T时平均降低16.6%和24.9%，在1.5 T时降低17.2%和28.1%；在纵向几何结构中，在0.5 T时降低10.56%和14.6%，在1.5 T时降低11.3%和16.3%。考虑到从线圈反射的散射光子，在LRBP和TRBP几何结构中，每个体素的平均相对剂量分别略有增加0.53%和0.32%。
将B0磁场平行于光子束轴定向，即LRBP几何结构，倾向于限制二次电子的径向扩散，从而降低对肺的剂量。在两种Linac-MR几何结构中观察到的剂量学问题，如外侧剂量分布的变化，在代表性乳腺计划中显著显示出对侧器官的剂量降低。此外，结果表明在1.5 T时两种几何结构的边缘剂量体积直方图曲线更尖锐，尤其是在LRBP配置中。
Zotero 插件