Han Silu, Yin Fang-Fang, Cai Jing
Medical Physics Graduate Program, Duke University Medical Center, Durham, NC, USA.
Department of Biomedical Engineering, University of Arizona, Tucson, AZ, USA.
J Appl Clin Med Phys. 2019 Feb;20(2):43-50. doi: 10.1002/acm2.12520. Epub 2019 Jan 29.
MRI-based treatment planning is a promising technique for liver stereotactic-body radiation therapy (SBRT) treatment planning to improve target volume delineation and reduce radiation dose to normal tissues. MR geometric distortion, however, is a source of potential error in MRI-based treatment planning. The aim of this study is to investigate dosimetric uncertainties caused by MRI geometric distortion in MRI-based treatment planning for liver SBRT.
The study was conducted using computer simulations. 3D MR geometric distortion was simulated using measured data in the literature. Planning MR images with distortions were generated by integrating the simulated 3D MR geometric distortion onto planning CT images. MRI-based treatment plans were then generated on the planning MR images with two dose calculation methods: (1) using original CT numbers; and (2) using organ-specific assigned CT numbers. Dosimetric uncertainties of various dose-volume-histogram parameters were determined as their differences between the simulated MRI-based plans and the original clinical CT-based plans for five liver SBRT cases.
The average simulated distortion for the five liver SBRT cases was 2.77 mm. In the case of using original CT numbers for dose calculation, the average dose uncertainties for target volumes and critical structures were <0.5 Gy, and the average target volume percentage at prescription dose uncertainties was 0.97%. In the case of using assigned CT numbers, the average dose uncertainties for target volumes and critical structures were <1.0 Gy, and the average target volume percentage at prescription dose uncertainties was 2.02%.
Dosimetric uncertainties caused by MR geometric distortion in MRI-based liver SBRT treatment planning was generally small (<1 Gy) when the distortion is 3 mm.
基于磁共振成像（MRI）的治疗计划是一种很有前景的技术，可用于肝脏立体定向体部放射治疗（SBRT）的治疗计划，以改善靶区体积勾画并减少对正常组织的辐射剂量。然而，MR几何畸变是基于MRI的治疗计划中潜在的误差来源。本研究的目的是调查在基于MRI的肝脏SBRT治疗计划中，由MRI几何畸变引起的剂量学不确定性。
本研究采用计算机模拟进行。使用文献中的测量数据模拟三维MR几何畸变。通过将模拟的三维MR几何畸变整合到计划CT图像上，生成带有畸变的计划MR图像。然后在计划MR图像上使用两种剂量计算方法生成基于MRI的治疗计划：（1）使用原始CT值；（2）使用器官特异性指定CT值。对于五例肝脏SBRT病例，确定各种剂量体积直方图参数的剂量学不确定性为基于模拟MRI的计划与原始基于临床CT的计划之间的差异。
五例肝脏SBRT病例的平均模拟畸变率为2.77毫米。在使用原始CT值进行剂量计算的情况下，靶区体积和关键结构的平均剂量不确定性<0.5 Gy，处方剂量不确定性时的平均靶区体积百分比为0.97%。在使用指定CT值的情况下，靶区体积和关键结构的平均剂量不确定性<1.0 Gy，处方剂量不确定性时的平均靶区体积百分比为2.02%。
当畸变率为3毫米时，基于MRI的肝脏SBRT治疗计划中由MR几何畸变引起的剂量学不确定性通常较小（<1 Gy）。
