Vásquez Osorio Eliana M, Kolkman-Deurloo Inger-Karine K, Schuring-Pereira Monica, Zolnay András, Heijmen Ben J M, Hoogeman Mischa S
Department of Radiation Oncology, Erasmus MC Cancer Institute, Rotterdam 3075, The Netherlands.
Med Phys. 2015 Jan;42(1):206-220. doi: 10.1118/1.4903300.
In the treatment of cervical cancer, large anatomical deformations, caused by, e.g., tumor shrinkage, bladder and rectum filling changes, organ sliding, and the presence of the brachytherapy (BT) applicator, prohibit the accumulation of external beam radiotherapy (EBRT) and BT dose distributions. This work proposes a structure-wise registration with vector field integration (SW+VF) to map the largely deformed anatomies between EBRT and BT, paving the way for 3D dose accumulation between EBRT and BT.
T2w-MRIs acquired before EBRT and as a part of the MRI-guided BT procedure for 12 cervical cancer patients, along with the manual delineations of the bladder, cervix-uterus, and rectum-sigmoid, were used for this study. A rigid transformation was used to align the bony anatomy in the MRIs. The proposed SW+VF method starts by automatically segmenting features in the area surrounding the delineated organs. Then, each organ and feature pair is registered independently using a feature-based nonrigid registration algorithm developed in-house. Additionally, a background transformation is calculated to account for areas far from all organs and features. In order to obtain one transformation that can be used for dose accumulation, the organ-based, feature-based, and the background transformations are combined into one vector field using a weighted sum, where the contribution of each transformation can be directly controlled by its extent of influence (scope size). The optimal scope sizes for organ-based and feature-based transformations were found by an exhaustive analysis. The anatomical correctness of the mapping was independently validated by measuring the residual distances after transformation for delineated structures inside the cervix-uterus (inner anatomical correctness), and for anatomical landmarks outside the organs in the surrounding region (outer anatomical correctness). The results of the proposed method were compared with the results of the rigid transformation and nonrigid registration of all structures together (AST).
The rigid transformation achieved a good global alignment (mean outer anatomical correctness of 4.3 mm) but failed to align the deformed organs (mean inner anatomical correctness of 22.4 mm). Conversely, the AST registration produced a reasonable alignment for the organs (6.3 mm) but not for the surrounding region (16.9 mm). SW+VF registration achieved the best results for both regions (3.5 and 3.4 mm for the inner and outer anatomical correctness, respectively). All differences were significant (p < 0.02, Wilcoxon rank sum test). Additionally, optimization of the scope sizes determined that the method was robust for a large range of scope size values.
The novel SW+VF method improved the mapping of large and complex deformations observed between EBRT and BT for cervical cancer patients. Future studies that quantify the mapping error in terms of dose errors are required to test the clinical applicability of dose accumulation by the SW+VF method.
在宫颈癌治疗中，诸如肿瘤缩小、膀胱和直肠充盈变化、器官滑动以及近距离放射治疗（BT）施源器的存在等因素所导致的较大解剖变形，会妨碍外照射放疗（EBRT）和BT剂量分布的累积。本研究提出一种基于结构的矢量场积分配准方法（SW+VF），用于在EBRT和BT之间映射大幅变形的解剖结构，为EBRT和BT之间的三维剂量累积铺平道路。
本研究使用了12例宫颈癌患者在EBRT前以及作为MRI引导BT程序一部分所采集的T2加权磁共振成像（T2w-MRI），以及膀胱、子宫颈和直肠乙状结肠的手动轮廓。采用刚性变换来对齐MRI中的骨骼解剖结构。所提出的SW+VF方法首先自动分割轮廓器官周围区域的特征。然后，使用内部开发的基于特征的非刚性配准算法独立配准每个器官和特征对。此外，计算背景变换以考虑远离所有器官和特征的区域。为了获得可用于剂量累积的单一变换，基于器官的、基于特征的和背景变换通过加权和组合成一个矢量场，其中每个变换的贡献可通过其影响范围（范围大小）直接控制。通过详尽分析确定了基于器官和基于特征变换的最佳范围大小。通过测量子宫颈内轮廓结构变换后的残余距离（内部解剖正确性）以及周围区域器官外解剖标志的残余距离（外部解剖正确性），独立验证了映射的解剖正确性。将所提方法的结果与刚性变换以及所有结构一起进行非刚性配准（AST）的结果进行比较。
刚性变换实现了良好的全局对齐（平均外部解剖正确性为4.3毫米），但未能对齐变形的器官（平均内部解剖正确性为22.4毫米）。相反，AST配准对器官产生了合理的对齐（6.3毫米），但对周围区域则不然（16.9毫米）。SW+VF配准在两个区域均取得了最佳结果（内部和外部解剖正确性分别为3.5和3.4毫米）。所有差异均具有统计学意义（p<0.02，Wilcoxon秩和检验）。此外，范围大小的优化确定该方法在大范围的范围大小值下具有鲁棒性。
新颖的SW+VF方法改善了宫颈癌患者EBRT和BT之间观察到的大而复杂变形的映射。未来需要开展根据剂量误差量化映射误差的研究，以测试SW+VF方法进行剂量累积的临床适用性。
