Potential underestimation of the internal target volume (ITV) from free-breathing CBCT
Potential underestimation of the internal target volume (ITV) from free-breathing CBCT
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DOI：
10.1118/1.3613153
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发表时间：
2011-08-01
期刊：
影响因子：
3.8
通讯作者：
Yin, Fang-Fang
中科院分区：
文献类型：
--
作者：
Vergalasova, Irina;Maurer, Jacqueline;Yin, Fang-Fang
目的：在向自由呼吸患者输送SBRT之前，通过将来自4DCT的计划内部靶体积（ITV）与机载自由呼吸锥束CT（FB-CBCT）图像对齐来进行定位。由于采集跨越多个呼吸周期，因此假定FB-CBCT图像也生成捕获全运动范围的ITV。然而，当吸气时间与呼气时间之比（I/E比）偏离1时，ITV可能被低估。因此，本研究的目的是调查的FB ITV从FB-CBCT scan.Methods产生的可变I/E比的影响：本研究采用了幻影和患者的成像数据。对于体模研究，使用不同的I/E比模拟了5个周期性呼吸循环。还选择了具有可变I/E比的六个患者呼吸周期。然后将所有轮廓编程到运动体模中进行成像，并修改为显示三个峰间运动幅度（0.5、1.0和2.0 cm）。使用直径为1.0和3.0 cm的两个球形靶对每个轮廓进行成像。用安装在现代直线加速器机架上的千伏（kV）成像仪的全机架旋转获取2D投影。使用标准滤波反投影重建算法从2D投影重建CBCT图像。体模研究的定量分析包括计算对比度沿着目标运动方向的变化，以及确定使用Canny边缘检测器提取的轮廓内的面积（与目标体积成比例）。对于患者研究，使用先前在研究性4D CBCT慢速机架成像方案下采集的投影数据生成FB-CBCT和4D CBCT图像。然后，手动轮廓从两个数据集（使用相同的窗口和水平）的定量comparation.Results：幻影研究表明，在对比度降低的ITV（对应于吸气）的下边缘的比率下降，为模拟和患者的呼吸周期。对于模拟体模呼吸周期，最小振幅的最大靶点的最小I/E比的对比度降低为27.6%，最大振幅的最小靶点的最小I/E比的对比度降低为89.7%。对于患者呼吸周期，这些数字分别为22.3%和94.0%。随着I/E比的降低，从目标轮廓内部提取的面积呈下降趋势。在患者研究中，与其相应的4D CBCT ITV相比，研究的两种肺肿瘤的FB-CBCT ITV均被低估。发现低估为40.1%的较小的肿瘤和24.2%的较大的tumor.Conclusions：ITV可能被低估的FB-CBCT图像时，患者的呼吸模式的特点是不同的长度的时间花费在吸气与呼气。在机载验证成像期间丢失完整的目标运动信息可能导致定位误差。(C)2011年美国医学物理学家协会。[DOI：10.1118/1.3613153]
Purpose: Localization prior to delivery of SBRT to free-breathing patients is performed by aligning the planning internal target volume (ITV) from 4DCT with an on-board free-breathing cone-beam CT (FB-CBCT) image. The FB-CBCT image is assumed to also generate an ITV that captures the full range of motion, due to the acquisition spanning multiple respiratory cycles. However, the ITV could potentially be underestimated when the ratio of time spent in inspiration versus time spent in expiration (I/E ratio) deviates from unity. Therefore, the aim of this study was to investigate the effect of variable I/E ratios on the FB ITV generated from a FB-CBCT scan.Methods: This study employed both phantom and patient imaging data. For the phantom study, five periodic respiratory cycles were simulated with different I/E ratios. Six patient respiratory cycles with variable I/E ratios were also selected. All profiles were then programmed into a motion phantom for imaging and modified to exhibit three peak-to-peak motion amplitudes (0.5, 1.0, and 2.0 cm). Each profile was imaged using two spherical targets with 1.0 and 3.0 cm diameters. 2D projections were acquired with full gantry rotation of a kiloVoltage (kV) imager mounted onto the gantry of a modern linear accelerator. CBCT images were reconstructed from 2D projections using a standard filtered back-projection reconstruction algorithm. Quantitative analyses for the phantom study included computing the change in contrast along the direction of target motion as well as determining the area (which is proportional to the target volume) inside of the contour extracted using a Canny edge detector. For the patient study, projection data that were previously acquired under an investigational 4D CBCT slow-gantry imaging protocol were used to generate both FB-CBCT and 4D CBCT images. Volumes were then manually contoured from both datasets (using the same window and level) for quantitative comparison.Results: The phantom study indicated a reduction in contrast at the inferior edge of the ITV (corresponding to inspiration) as the ratio decreased, for both simulated and patient respiratory cycles. For the simulated phantom respiratory cycles, the contrast reduction of the smallest I/E ratio was 27.6% for the largest target with the smallest amplitude and 89.7% for the smallest target with the largest amplitude. For patient respiratory cycles, these numbers were 22.3% and 94.0%, respectively. The extracted area from inside of the target contours showed a decreasing trend as the I/E ratio decreased. In the patient study, the FB-CBCT ITVs of both lung tumors studied were underestimated when compared with their corresponding 4D CBCT ITV. The underestimations found were 40.1% for the smaller tumor and 24.2% for the larger tumor.Conclusions: The ITV may be underestimated in a FB-CBCT image when a patient's respiratory pattern is characterized by a disparate length of time spent in inspiration versus expiration. Missing the full target motion information during on-board verification imaging may result in localization errors. (C) 2011 American Association of Physicists in Medicine. [DOI: 10.1118/1.3613153]