Department of Imaging Physics, The University of Texas MD Anderson Cancer Center, Houston, Texas 77030, USA.
Med Phys. 2010 Nov;37(11):5811-20. doi: 10.1118/1.3504605.
Four-dimensional computed tomography (4D-CT) is commonly used to account for respiratory motion of target volumes in radiotherapy to the thorax. From the 4D-CT acquisition, a maximum-intensity projection (MIP) image set can be created and used to help define the tumor motion envelope or the internal gross tumor volume (iGTV). The purpose of this study was to quantify the differences in automatically contoured target volumes for usage in the delivery of stereotactic body radiation therapy using MIP data sets generated from one of the four methods: (1) 4D-CT phase-binned (PB) based on retrospective phase calculations, (2) 4D-CT phase-corrected phase-binned (PC-PB) based on motion extrema, (3) 4D-CT amplitude-binned (AB), and (4) cine CT built from all available images.
MIP image data sets using each of the four methods were generated for a cohort of 28 patients who had prior thoracic 4D-CT scans that exhibited lung tumor motion of at least 1 cm. Each MIP image set was automatically contoured on commercial radiation treatment planning system. Margins were added to the iGTV to observe differences in the final simulated planning target volumes (PTVs).
For all patients, the iGTV measured on the MIP generated from the entire cine CT data set (iGTVcine) was the largest. Expressed as a percentage of iGTVcine, 4D-CT iGTV (all sorting methods) ranged from 83.8% to 99.1%, representing differences in the absolute volume ranging from 0.02 to 4.20 cm3; the largest average and range of 4D-CT iGTV measurements was from the PC-PB data set. Expressed as a percentage of PTVcine (expansions applied to iGTVeine), the 4D-CT PTV ranged from 87.6% to 99.6%, representing differences in the absolute volume ranging from 0.08 to 7.42 cm3. Regions of the measured respiratory waveform corresponding to a rapid change of phase or amplitude showed an increased susceptibility to the selection of identical images for adjacent bins. Duplicate image selection was most common in the AB implementation, followed by the PC-PB method. The authors also found that the image associated with the minimum amplitude measurement did not always correlate with the image that showed maximum tumor motion extent.
The authors identified cases in which the MIP generated from a 4D-CT sorting process under-represented the iGTV by more than 10% or up to 4.2 cm3 when compared to the iGTVcine. They suggest utilization of a MIP generated from the full cine CT data set to ensure maximum inclusive tumor extent.
四维计算机断层扫描（4D-CT）常用于在胸部放射治疗中考虑靶区的呼吸运动。从 4D-CT 采集，可以创建最大强度投影（MIP）图像集，并用于帮助定义肿瘤运动包络或内部大体肿瘤体积（iGTV）。本研究的目的是量化在使用以下四种方法之一生成的 MIP 数据集自动勾画靶区体积时的差异，用于立体定向体部放射治疗：（1）基于回顾性相位计算的 4D-CT 相位分箱（PB），（2）基于运动极值的 4D-CT 相位校正相位分箱（PC-PB），（3）4D-CT 幅度分箱（AB），和（4）从所有可用图像构建的电影 CT。
对于至少有 1 厘米肺肿瘤运动的 28 例患者的队列，使用上述四种方法中的每一种生成 MIP 图像数据集。每个 MIP 图像集都在商业放射治疗计划系统上自动勾画。在 iGTV 上添加边缘以观察最终模拟计划靶区体积（PTV）的差异。
对于所有患者，在从整个电影 CT 数据集生成的 MIP 上测量的 iGTV（iGTVcine）最大。以 iGTVcine 的百分比表示，4D-CT iGTV（所有排序方法）范围为 83.8%至 99.1%，表示绝对体积的差异范围为 0.02 至 4.20cm3；PC-PB 数据集的 4D-CT iGTV 测量值最大且范围最大。以 PTVcine 的百分比表示（应用于 iGTVcine 的扩展），4D-CT PTV 范围为 87.6%至 99.6%，表示绝对体积的差异范围为 0.08 至 7.42cm3。与相位或幅度的快速变化相对应的测量呼吸波形区域容易选择相邻的相同图像。在 AB 实现中，最常见的是重复图像选择，其次是 PC-PB 方法。作者还发现，与显示最大肿瘤运动范围的图像相关联的图像不一定与显示最大肿瘤运动范围的图像相关联。
作者发现，与 iGTVcine 相比，4D-CT 排序过程生成的 MIP 代表 iGTV 时，代表 iGTV 的图像少 10%或多达 4.2cm3。他们建议使用从完整电影 CT 数据集生成的 MIP 以确保包含最大的肿瘤范围。
