Department of Electronics, Information and Bioengineering, Politecnico di Milano, Via Ponzio 34/5, 20133 Milano, Italy; Department of Medical Physics, Faculty of Physics, Ludwig-Maximilians-Universität München, Am Coulombwall 1, 85748 Garching bei München, Germany.
Department of Electronics, Information and Bioengineering, Politecnico di Milano, Via Ponzio 34/5, 20133 Milano, Italy.
Z Med Phys. 2022 Feb;32(1):85-97. doi: 10.1016/j.zemedi.2020.10.002. Epub 2020 Nov 7.
In a radiation therapy workflow based on Magnetic Resonance Imaging (MRI), dosimetric errors may arise due to geometric distortions introduced by MRI. The aim of this study was to quantify the dosimetric effect of system-dependent geometric distortions in an MRI-only workflow for proton therapy applied at extra-cranial sites. An approach was developed, in which computed tomography (CT) images were distorted using an MRI displacement map, which represented the MR distortions in a spoiled gradient-echo sequence due to gradient nonlinearities and static magnetic field inhomogeneities. A retrospective study was conducted on 4DCT/MRI digital phantoms and 18 4DCT clinical datasets of the thoraco-abdominal site. The treatment plans were designed and separately optimized for each beam in a beam specific Planning Target Volume on the distorted CT, and the final dose distribution was obtained as the average. The dose was then recalculated in undistorted CT using the same beam geometry and beam weights. The analysis was performed in terms of Dose Volume Histogram (DVH) parameters. No clinically relevant dosimetric impact was observed on organs at risk, whereas in the target structure, geometric distortions caused statistically significant variations in the planned dose DVH parameters and dose homogeneity index (DHI). The dosimetric variations in the target structure were smaller in abdominal cases (ΔD, ΔD, and ΔD all below 0.1% and ΔDHI below 0.003) compared to the lung cases. Indeed, lung patients with tumors isolated inside lung parenchyma exhibited higher dosimetric variations (ΔD≥0.3%, ΔD≥15.9%, ΔD≥3.3% and ΔDHI≥0.102) than lung patients with tumor close to soft tissue (ΔD≤0.4%, ΔD≤5.6%, ΔD≤0.9% and ΔDHI≤0.027) potentially due to higher density variations along the beam path. Results suggest the potential applicability of MRI-only proton therapy, provided that specific analysis is applied for isolated lung tumors.
在基于磁共振成像（MRI）的放射治疗工作流程中，由于 MRI 引入的几何变形，可能会出现剂量学误差。本研究的目的是量化在仅用于 MRI 的质子治疗工作流程中，由于梯度非线性和静态磁场不均匀性导致的各向异性梯度回波序列中 MRI 变形引起的剂量学效应，应用于颅外部位。我们开发了一种方法，使用 CT 图像通过 MRI 位移图进行变形，该位移图表示由于梯度非线性和静态磁场不均匀性导致的各向异性梯度回波序列中的 MR 变形。对 4DCT/MRI 数字体模和胸腹部的 18 个 4DCT 临床数据集进行了回顾性研究。在变形 CT 上的特定光束计划靶区（PTV）的每个光束中分别设计和单独优化治疗计划，并将最终剂量分布作为平均值获得。然后使用相同的光束几何形状和光束权重在未变形的 CT 上重新计算剂量。分析是根据剂量体积直方图（DVH）参数进行的。在危及器官方面未观察到与临床相关的剂量学影响，而在靶结构中，几何变形导致计划剂量 DVH 参数和剂量均匀性指数（DHI）的统计学显著变化。与肺部病例相比，腹部病例的靶结构剂量变化较小（ΔD、ΔD 和 ΔD 均低于 0.1%，ΔDHI 低于 0.003）。实际上，与肿瘤靠近软组织的肺部患者（ΔD≤0.4%，ΔD≤5.6%，ΔD≤0.9%和ΔDHI≤0.027）相比，肿瘤完全位于肺实质内的肺部患者的剂量学变化更高（ΔD≥0.3%，ΔD≥15.9%，ΔD≥3.3%和ΔDHI≥0.102），这可能是由于沿射束路径的密度变化更高所致。结果表明，只要对孤立性肺肿瘤进行特定分析，就有可能应用仅使用 MRI 的质子治疗。
