Deurloo Kirsten E I, Steenbakkers Roel J H M, Zijp Lambert J, de Bois Josien A, Nowak Peter J C M, Rasch Coen R N, van Herk Marcel
Radiotherapy Department, The Netherlands Cancer Institute/Antoni van Leeuwenhoek Hospital, Amsterdam, The Netherlands.
Int J Radiat Oncol Biol Phys. 2005 Jan 1;61(1):228-38. doi: 10.1016/j.ijrobp.2004.09.023.
The prostate is known to translate and rotate under influence of rectal filling changes and many studies have addressed the magnitude of these motions. However, prostate shape variations also have been reported. For image-guided radiotherapy, it is essential to know the relative magnitude of translations, rotations, and shape variation so that the most appropriate correction strategy can be chosen. However, no quantitative analysis of shape variation has been performed. It is, therefore, the purpose of this article to develop a method to determine shape variation of complex organs and apply it to determine shape variation during external beam radiotherapy of a GTV (gross tumor volume) consisting of prostate and seminal vesicles.
For this study, the data of 19 patients with prostate cancer were used. Each patient received a planning computed tomography (CT) scan and 8-12 (11 on average) repeat CT scans that were made during the course of conformal radiotherapy. One observer delineated the GTV in all scans, and volume variations were measured. After matching the GTVs for each patient for translation and rotation, a coverage probability matrix was constructed and the 50% isosurface was taken to determine the average GTV surface. Perpendicular distances between the average GTV and the individual GTVs were calculated for each point of the average GTV, and their variation was expressed in terms of local standard deviation (SD). The local SDs of the shape variation of all 19 patients were mapped onto a reference case by matching and morphing of the individual average GTVs. Repeated delineation of the GTV was done for 6 patients to determine intraobserver variation. Finally, the measured shape variation was corrected for intraobserver variation to estimate the "real" shape variation.
No significant variations in GTV volume were observed. The measured shape variation (including delineation variation) was largest at the tip of the vesicles (SD = 2.0 mm), smallest at the left and right side of the prostate (SD = 1.0 mm), and average elsewhere (SD = 1.5 mm). At the left, right, and cranial sides of the prostate, the intraobserver variation was of the same order of magnitude as the measured shape variation; elsewhere it was smaller. However, the accuracy of the estimated SD for intraobserver variation was about half of the accuracy of the estimated SD for the measured shape variation, giving an overall uncertainty of maximum 0.6 mm SD in the estimate of the "real" shape variation. The "real" shape variation was small at the left, right, and cranial side of the prostate (SD <0.5 mm) and between 0.5 mm and 1.6 mm elsewhere.
We developed a method to quantify shape variation of organs with a complex shape and applied it to a GTV consisting of prostate and seminal vesicles. Deformation of prostate and seminal vesicles during the course of radiotherapy is small (relative to organ motion). Therefore, it is a valid approximation in image-guided radiotherapy of prostate cancer, in first order, to correct only for setup errors and organ motion. Prostate and seminal vesicles deformation can be considered as a second-order effect.
已知前列腺会在直肠充盈变化的影响下发生平移和旋转，许多研究已探讨了这些运动的幅度。然而，也有报道称前列腺形状存在变化。对于图像引导放射治疗而言，了解平移、旋转和形状变化的相对幅度至关重要，以便能够选择最合适的校正策略。然而，尚未对形状变化进行定量分析。因此，本文的目的是开发一种确定复杂器官形状变化的方法，并将其应用于确定由前列腺和精囊组成的大体肿瘤体积（GTV）在体外束放射治疗期间的形状变化。
本研究使用了19例前列腺癌患者的数据。每位患者均接受了一次计划计算机断层扫描（CT）以及在适形放疗过程中进行的8 - 12次（平均11次）重复CT扫描。一名观察者在所有扫描图像中勾画出GTV，并测量体积变化。在对每位患者的GTV进行平移和旋转匹配后，构建覆盖概率矩阵，并采用50%等值面来确定平均GTV表面。针对平均GTV的每个点，计算其与各个GTV之间的垂直距离，并以局部标准差（SD）表示其变化。通过对各个平均GTV进行匹配和变形，将所有19例患者形状变化的局部标准差映射到一个参考病例上。对6例患者的GTV进行重复勾画以确定观察者内变异。最后，对测量的形状变化进行观察者内变异校正，以估计“真实”形状变化。
未观察到GTV体积有显著变化。测量的形状变化（包括勾画变异）在精囊尖端最大（SD = 2.0 mm），在前列腺左右两侧最小（SD = 1.0 mm），其他部位平均（SD = 1.5 mm）。在前列腺的左侧、右侧和头侧，观察者内变异与测量的形状变化处于同一数量级；其他部位则较小。然而，观察者内变异估计标准差的准确性约为测量形状变化估计标准差准确性的一半，在估计“真实”形状变化时，总体不确定性最大为0.6 mm SD。前列腺左侧、右侧和头侧的“真实”形状变化较小（SD <0.5 mm），其他部位在0.5 mm至1.6 mm之间。
我们开发了一种量化复杂形状器官形状变化的方法，并将其应用于由前列腺和精囊组成的GTV。放疗过程中前列腺和精囊的变形较小（相对于器官运动）。因此，在前列腺癌的图像引导放射治疗中，一阶近似下仅校正摆位误差和器官运动是有效的。前列腺和精囊的变形可视为二阶效应。
