MAGNETIC RESONANCE IMAGING BASED TREATMENT PLANNING IN IMRT AND VMAT FOR HEAD REGION

MRI can improve delineation of tumor and normal tissues for radiation treatment planning in brain tumor. However, MRI cannot provide radiologic properties information as in the case of CT image. The approach can solve this problem by bulk density method. This method works by overriding the original pixel value over the interested area with average bulk density. The purpose of this study is to evaluate the dosimetric accuracy in MRI based compared with CT base treatment planning. Twenty eight brain tumor patients were scanned both CT and MRI simulator with the same position and then imported both of them to Radiotherapy Treatment Planning (RTP) unit. Registration images and delineation of tumor and OARs were created by radiation oncologist while bone and air cavity were defined with auto-segmentation in treatment planning. Determination of 3 types of bulk density: First, average individual bulk density: the average CT number of all slices for PTV, bone and air for each patient were undertaken. Second, average mean bulk density: the average individual bulk density for all slices and all patients for PTV, bone and air were calculated. And third, ICRU bulk density: recommendation from ICRU number 46 for PTV, bone and air were employed. CT full density plan were created in IMRT and VMAT as a reference plan of this study. MRI with 3 types of bulk density plans and MRI with water equivalent plan were created in both IMRT and VMAT. The plans were compared using D_95% in the dose volume histogram. The result of IMRT and VMAT plan, show that all the dose differences from CT full density plan of MRI average individual and average mean bulk density plan are within 0.27% and 1.54% for average and maximum dose difference, respectively, and the dose difference of MRI with ICRU bulk density plan were within 0.38% and 0.93% for average and maximum dose difference, respectively, The water equivalent plans were within 1.68% and 3.83% for average and maximum dose difference, respectively. The treatment planning of head region using MR image was quite accurate with bulk density method. The dose differences in both of average individual and average mean plan were comparable so the average bulk density for this group of study could be employed in the head planning instead of determining bulk density for individual. The ICRU bulk density can also be used in head planning. The MR water equivalent plan may not be suitable to use in head region.

磁共振成像（Magnetic Resonance Imaging, MRI）可提升脑肿瘤放射治疗计划中肿瘤与正常组织的勾画精度。然而，磁共振成像无法如计算机断层扫描（Computed Tomography, CT）那般提供放射学特性信息。本研究采用体密度法（bulk density method）解决该问题：通过用平均体密度替换感兴趣区内的原始像素值来实现。本研究旨在评估基于磁共振成像的治疗计划相较于基于CT的治疗计划的剂量学准确性。研究纳入28例脑肿瘤患者，采用相同体位完成CT与MRI模拟扫描，并将两组图像导入放射治疗计划系统（Radiotherapy Treatment Planning, RTP）。由放射肿瘤医师完成图像配准及肿瘤与危及器官（Organs At Risk, OARs）的勾画，骨组织与气腔则通过治疗计划系统中的自动分割工具完成定义。本研究共定义三类体密度：① 个体平均体密度：计算每例患者的大体肿瘤靶区（Planning Target Volume, PTV）、骨组织与气腔所有扫描层面的CT值平均值；② 总体平均体密度：计算所有患者的大体肿瘤靶区、骨组织与气腔所有扫描层面的个体平均体密度的总体平均值；③ ICRU体密度：采用国际辐射单位与测量委员会（International Commission on Radiation Units and Measurements, ICRU）第46号报告推荐的大体肿瘤靶区、骨组织与气腔体密度标准。本研究以调强放射治疗（Intensity-Modulated Radiation Therapy, IMRT）与容积旋转调强放疗（Volumetric Modulated Arc Therapy, VMAT）模式下的全CT密度计划作为参考基准计划。同时分别基于三种体密度方法与水等效方法，构建IMRT与VMAT模式下的MRI治疗计划。采用剂量体积直方图（Dose Volume Histogram, DVH）中的D_95%指标对各计划进行对比分析。IMRT与VMAT计划的对比结果显示：相较于全CT密度参考计划，基于个体平均体密度与总体平均体密度的MRI计划，其剂量偏差的平均值与最大值分别不超过0.27%与1.54%；基于ICRU体密度的MRI计划，其剂量偏差的平均值与最大值分别不超过0.38%与0.93%；而水等效计划的剂量偏差平均值与最大值分别不超过1.68%与3.83%。采用体密度法的头部区域MRI放射治疗计划具备良好的剂量准确性。个体平均体密度计划与总体平均体密度计划的剂量偏差无显著差异，因此针对本研究的患者群体，可采用总体平均体密度替代个体体密度计算，用于头部区域的放射治疗计划。ICRU体密度同样可应用于头部区域的放射治疗计划。而MRI水等效计划则不适用于头部区域的放射治疗计划。