Beekman Chris, van Beek Suzanne, Stam Jikke, Sonke Jan-Jakob, Remeijer Peter
Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, The Netherlands.
Med Phys. 2020 Sep;47(9):3852-3860. doi: 10.1002/mp.14349. Epub 2020 Jul 16.
To generate a series of physiologically plausible cervix CTVs by biomechanically modeling organ deformation as a consequence of bladder filling. This series can serve as planning CTVs for radiotherapy treatment of cervical cancer patients using a library of plans (LoP) strategy.
The model was constructed based on the full and empty bladder scans of 20 cervical cancer patients, for which the bladder, rectum and the clinical target volume (CTV) of the cervix were delineated. Finite element modeling (FEM) was used to deform empty to full bladder anatomy. This deformation comprised two steps. In the first step, the surfaces of the bladder and rectum of the empty bladder anatomy were explicitly deformed to the full bladder anatomy and imported as enforced displacements into the biomechanical model. These surface displacements cause volumetric deformations of the bladder, rectum and cervix CTV meshes, dictated by their respective elastic properties and the type of contact among them. In the second step, the residual offset between the simulated and target CTV was corrected by an additional thin plate spline warp. Intermediate structural outputs of a linear superposition of the biomechanical and residual warp then constituted the library of CTVs for each patient. The residual warp was minimized by optimizing the FEM parameters over the 20 patients. Finally, the model was tested for nine healthy volunteers for which repeat MR scans were available as the bladder filled from empty to full. Small and large movers were identified depending on the extent of CTV motion, and analyzed separately. The proposed method was compared against the method currently used in our institute, in which intermediate structures are linearly interpolated between full and empty bladder anatomy, using a thin plate spline warp. The comparison metrics used were the ability to preserve CTV volume throughout the deformation, and residual offsets between repeat and library CTV.
Optimal model parameters were found to be compatible with published values. While for the current method, the median CTV volume shrunk by 4% for large movers halfway the deformation (and by up to 10% for individual cases), the proposed FEM-based method preserved CTV volumes throughout the deformation. Regional residual errors between repeat and library CTV reduced by up to 3 mm when averaged over the group of large movers. For individual cases this regional error reduction could be as large as 8 mm.
We developed a robust and automatic method to create a patient-specific FEM-based LoP. The FEM-based method resulted in more accurate library of planning CTVs as compared to the current method, with the greatest improvements observed for patients with large CTV motion. The biomechanical model simulates volumetric deformations from empty to full bladder anatomy, paving the way for dose accumulation in an LoP setting.
通过对膀胱充盈导致的器官变形进行生物力学建模，生成一系列生理上合理的子宫颈临床靶体积（CTV）。该系列可作为使用计划库（LoP）策略对宫颈癌患者进行放射治疗的计划CTV。
基于20例宫颈癌患者的膀胱充盈和排空扫描构建模型，在扫描图像上勾勒出膀胱、直肠和子宫颈的临床靶体积（CTV）。采用有限元建模（FEM）将排空膀胱的解剖结构变形为充盈膀胱的解剖结构。这种变形包括两个步骤。第一步，将排空膀胱解剖结构的膀胱和直肠表面明确变形为充盈膀胱的解剖结构，并作为强制位移导入生物力学模型。这些表面位移会导致膀胱、直肠和子宫颈CTV网格的体积变形，这由它们各自的弹性特性以及它们之间的接触类型决定。第二步，通过额外的薄板样条变形校正模拟CTV与目标CTV之间的残余偏移。生物力学变形和残余变形线性叠加的中间结构输出构成了每位患者的CTV库。通过在20例患者中优化有限元参数，使残余变形最小化。最后，对9名健康志愿者进行了测试，这些志愿者有从膀胱排空到充盈的重复磁共振扫描数据。根据CTV运动程度确定了大运动者和小运动者，并分别进行分析。将所提出的方法与我们研究所目前使用的方法进行比较，目前的方法是在排空和充盈膀胱解剖结构之间使用薄板样条变形进行线性插值。使用的比较指标是在整个变形过程中保持CTV体积的能力，以及重复CTV与库CTV之间的残余偏移。
发现最佳模型参数与已发表的值兼容。对于当前方法，在变形过程中，大运动者的CTV体积中位数在变形中期缩小了4%（个别情况缩小高达10%），而所提出的基于有限元的方法在整个变形过程中保持了CTV体积。在大运动者组中平均计算时，重复CTV与库CTV之间的区域残余误差减少了多达3毫米。对于个别情况，这种区域误差减少可能高达8毫米。
我们开发了一种强大的自动方法来创建基于患者特定有限元的LoP。与当前方法相比，基于有限元的方法产生了更准确的计划CTV库，对于CTV运动较大的患者观察到最大的改进。生物力学模型模拟了从排空到充盈膀胱解剖结构的体积变形，为在LoP设置中进行剂量累积铺平了道路。
