Department of Radiation Oncology, University of Michigan, Ann Arbor, MI 48109, United States of America.
Department of Radiation Oncology, Stanford University, Palo Alto, CA 94304, United States of America.
Phys Med Biol. 2021 Apr 12;66(8). doi: 10.1088/1361-6560/abef42.
Abdominal organ motions introduce geometric uncertainties to gastrointestinal radiotherapy. This study investigated slow drifting motion induced by changes of internal anatomic organ arrangements using a 3D radial MRI sequence with a scan length of 20 min. Breathing motion and cyclic GI motion were first removed through multi-temporal resolution image reconstruction. Slow drifting motion analysis was performed using an image time series consisting of 72 image volumes with a temporal sampling rate of 17 s. B-spline deformable registration was performed to align image volumes of the time series to a reference volume. The resulting deformation fields were used for motion velocity evaluation and patient-specific motion model construction through principal component analysis (PCA). Geometric uncertainties introduced by slow drifting motion were assessed by Hausdorff distances between unions of organs at risk (OARs) at different motion states and reference OAR contours as well as probabilistic distributions of OARs predicted using the PCA model. Thirteen examinations from 11 patients were included in this study. The averaged motion velocities ranged from 0.8 to 1.9 mm min, 0.7 to 1.6 mm min, 0.6 to 2.0 mm minand 0.7 to 1.4 mm minfor the small bowel, colon, duodenum and stomach respectively; the averaged Hausdorff distances were 5.6 mm, 5.3 mm, 5.1 mm and 4.6 mm. On average, a margin larger than 4.5 mm was needed to cover a space with OAR occupancy probability higher than 55%. Temporal variations of geometric uncertainties were evaluated by comparing across four 5 min sub-scans extracted from the full scan. Standard deviations of Hausdorff distances across sub-scans were less than 1 mm for most examinations, indicating stability of relative margin estimates from separate time windows. These results suggested slow drifting motion of GI organs is significant and geometric uncertainties introduced by such motion should be accounted for during radiotherapy planning and delivery.
腹部器官运动为胃肠道放射治疗带来了几何不确定性。本研究使用一种扫描长度为 20 分钟的 3D 放射状 MRI 序列，研究了由于内部解剖器官排列变化引起的缓慢漂移运动。通过多时相分辨率图像重建，首先去除呼吸运动和周期性 GI 运动。通过包含 72 个图像卷的图像时间序列进行缓慢漂移运动分析，时间采样率为 17 秒。使用 B 样条变形配准将时间序列的图像卷与参考体积对齐。通过主成分分析 (PCA) 对变形场进行运动速度评估和患者特定运动模型构建。通过 PCA 模型预测 OAR 位置的概率分布，评估由缓慢漂移运动引起的几何不确定性，通过不同运动状态和参考 OAR 轮廓之间的 OAR 联合的 Hausdorff 距离以及 OAR 的概率分布来评估。本研究纳入了 11 名患者的 13 次检查。平均运动速度分别为小肠、结肠、十二指肠和胃的 0.8 至 1.9 毫米/分钟、0.7 至 1.6 毫米/分钟、0.6 至 2.0 毫米/分钟和 0.7 至 1.4 毫米/分钟；平均 Hausdorff 距离分别为 5.6 毫米、5.3 毫米、5.1 毫米和 4.6 毫米。平均而言，需要超过 4.5 毫米的边界来覆盖 OAR 占有率高于 55%的空间。通过比较从完整扫描中提取的四个 5 分钟子扫描来评估几何不确定性的时间变化。对于大多数检查，子扫描之间的 Hausdorff 距离标准差小于 1 毫米，表明来自单独时间窗口的相对边界估计的稳定性。这些结果表明，胃肠道器官的缓慢漂移运动是显著的，在放射治疗计划和实施过程中应考虑这种运动引起的几何不确定性。
