Velec Michael, Moseley Joanne L, Brock Kristy K
Radiation Medicine Program, Princess Margaret Cancer Centre, Toronto, Ontario, Canada; Institute of Medical Science, University of Toronto, Toronto, Ontario, Canada.
Radiation Medicine Program, Princess Margaret Cancer Centre, Toronto, Ontario, Canada.
Pract Radiat Oncol. 2014 May-Jun;4(3):160-166. doi: 10.1016/j.prro.2013.07.001. Epub 2013 Aug 8.
Establishing the time-weighted mean respiratory position in the liver is challenging due to poor tumor contrast on 4-dimensional (4D) imaging. The purpose of this study is to validate simplified strategies in determining the mean position of liver tumors for radiation therapy planning, and quantify the potential for planning target volume (PTV) reduction.
Full, 10-phase 4D computed tomography (CT) data sets from 10 liver radiation therapy patients were analyzed to compare 2 techniques. First, a mid-ventilation CT was chosen from the initial reconstruction of the 4DCT. This was based on the minimum displacement of the diaphragm at each phase relative to its mean respiratory position, calculated using rigid registration over all 4DCT phases. Second, the exhale 4DCT was deformed to the inhale 4DCT using biomechanical-based deformable registration. The diaphragm's mean cranio-caudal position in the respiratory cycle (normalized as a percentage relative to exhale) was applied to the exhale-to-inhale deformation map assuming a linear trajectory to reconstruct a mid-position CT. These strategies were compared with the time-weighted mean respiratory position, calculated with deformable registration over all 10 4DCT phases. PTVs incorporating respiratory motion were then compared for 2 planning strategies: exhale 4DCT using the internal target volume (ITV), or mid-position CT using dose-probability margins.
Compared with the mean respiratory tumor position, the mid-ventilation CT and mid-position CT had mean (maximum) tumor vector errors of 1.0 ± 0.5 (2.1) mm and 0.6 ± 0.3 (1.4) mm, respectively, within the image resolution. Compared with ITV-based PTV, dose-probability PTV reduced the irradiated volume by 34% ± 7% on average, up to 43%.
Simplified strategies to select a mid-ventilation CT or reconstruct a mid-position CT for the liver were validated with respect to the mean respiratory position. These data sets require significantly smaller PTVs, potentially allowing for dose-escalated liver stereotactic body radiation therapy to improve local control.
由于四维（4D）成像中肿瘤对比度较差，确定肝脏的时间加权平均呼吸位置具有挑战性。本研究的目的是验证在放射治疗计划中确定肝脏肿瘤平均位置的简化策略，并量化计划靶体积（PTV）缩小的潜力。
分析了10例肝脏放射治疗患者的完整10期4D计算机断层扫描（CT）数据集，以比较两种技术。首先，从4DCT的初始重建中选择一次中期通气CT。这是基于使用所有4DCT期的刚性配准计算出的每个期膈肌相对于其平均呼吸位置的最小位移。其次，使用基于生物力学的可变形配准将呼气4DCT变形为吸气4DCT。假设线性轨迹，将呼吸周期中膈肌的平均头脚位置（相对于呼气归一化为百分比）应用于呼气到吸气变形图，以重建中间位置CT。将这些策略与通过对所有10个4DCT期进行可变形配准计算出的时间加权平均呼吸位置进行比较。然后比较了两种计划策略的包含呼吸运动的PTV：使用内部靶体积（ITV）的呼气4DCT或使用剂量概率边缘的中间位置CT。
与平均呼吸肿瘤位置相比，中期通气CT和中间位置CT在图像分辨率内的平均（最大）肿瘤向量误差分别为1.0±0.5（2.1）mm和0.6±0.3（1.4）mm。与基于ITV的PTV相比，剂量概率PTV平均将照射体积减少了34%±7%，最高可达43%。
关于平均呼吸位置，验证了选择中期通气CT或重建肝脏中间位置CT的简化策略。这些数据集需要显著更小的PTV，可能允许进行剂量递增的肝脏立体定向体部放射治疗以改善局部控制。
