Faculty of Electrical Engineering, University of Ljubljana, Ljubljana, Slovenia.
Department of Computer Science, University of Copenhagen, Copenhagen, Denmark.
Med Phys. 2024 Mar;51(3):2175-2186. doi: 10.1002/mp.16924. Epub 2024 Jan 17.
Accurate and consistent contouring of organs-at-risk (OARs) from medical images is a key step of radiotherapy (RT) cancer treatment planning. Most contouring approaches rely on computed tomography (CT) images, but the integration of complementary magnetic resonance (MR) modality is highly recommended, especially from the perspective of OAR contouring, synthetic CT and MR image generation for MR-only RT, and MR-guided RT. Although MR has been recognized as valuable for contouring OARs in the head and neck (HaN) region, the accuracy and consistency of the resulting contours have not been yet objectively evaluated.
To analyze the interobserver and intermodality variability in contouring OARs in the HaN region, performed by observers with different level of experience from CT and MR images of the same patients.
In the final cohort of 27 CT and MR images of the same patients, contours of up to 31 OARs were obtained by a radiation oncology resident (junior observer, JO) and a board-certified radiation oncologist (senior observer, SO). The resulting contours were then evaluated in terms of interobserver variability, characterized as the agreement among different observers (JO and SO) when contouring OARs in a selected modality (CT or MR), and intermodality variability, characterized as the agreement among different modalities (CT and MR) when OARs were contoured by a selected observer (JO or SO), both by the Dice coefficient (DC) and 95-percentile Hausdorff distance (HD ).
The mean (±standard deviation) interobserver variability was 69.0 ± 20.2% and 5.1 ± 4.1 mm, while the mean intermodality variability was 61.6 ± 19.0% and 6.1 ± 4.3 mm in terms of DC and HD , respectively, across all OARs. Statistically significant differences were only found for specific OARs. The performed MR to CT image registration resulted in a mean target registration error of 1.7 ± 0.5 mm, which was considered as valid for the analysis of intermodality variability.
The contouring variability was, in general, similar for both image modalities, and experience did not considerably affect the contouring performance. However, the results indicate that an OAR is difficult to contour regardless of whether it is contoured in the CT or MR image, and that observer experience may be an important factor for OARs that are deemed difficult to contour. Several of the differences in the resulting variability can be also attributed to adherence to guidelines, especially for OARs with poor visibility or without distinctive boundaries in either CT or MR images. Although considerable contouring differences were observed for specific OARs, it can be concluded that almost all OARs can be contoured with a similar degree of variability in either the CT or MR modality, which works in favor of MR images from the perspective of MR-only and MR-guided RT.
准确且一致地勾画医学图像中的危及器官（OAR）是放射治疗（RT）癌症治疗计划的关键步骤。大多数勾画方法都依赖于计算机断层扫描（CT）图像，但强烈推荐整合互补的磁共振（MR）模式，特别是从 OAR 勾画、合成 CT 和用于仅 MR 放疗和 MR 引导放疗的 MR 图像生成的角度来看。尽管 MR 已经被认为对勾画头颈部（HaN）区域的 OAR 很有价值，但勾画结果的准确性和一致性尚未得到客观评估。
分析由具有不同经验水平的观察者从同一患者的 CT 和 MR 图像勾画 HaN 区域 OAR 时的观察者间和模态间变异性。
在最终的 27 例 CT 和 MR 图像的队列中，由一名放射肿瘤学住院医师（初级观察者，JO）和一名经过董事会认证的放射肿瘤学家（高级观察者，SO）勾画多达 31 个 OAR 的轮廓。然后，根据观察者间变异性（JO 和 SO 在选定模式（CT 或 MR）中勾画 OAR 时的一致性）和模态间变异性（JO 或 SO 勾画 OAR 时不同模态之间的一致性），使用 Dice 系数（DC）和 95%Hausdorff 距离（HD ）来评估勾画结果。
在所有 OAR 中，观察者间变异性的平均值（±标准差）分别为 69.0±20.2%和 5.1±4.1mm，而模态间变异性的平均值分别为 61.6±19.0%和 6.1±4.3mm，分别为 DC 和 HD 。仅在特定 OAR 中发现了统计学显著差异。所进行的 MR 到 CT 图像配准导致目标配准误差的平均值为 1.7±0.5mm，这被认为足以分析模态间变异性。
总体而言，两种成像模式的勾画变异性相似，经验并没有显著影响勾画性能。然而，结果表明，无论在 CT 还是 MR 图像中勾画，OAR 都难以勾画，观察者的经验可能是 OAR 难以勾画的一个重要因素。在产生的变异性中，一些差异也可以归因于对指南的遵守，特别是对于在 CT 或 MR 图像中都没有明显可见性或没有明显边界的 OAR。尽管在特定的 OAR 中观察到了相当大的勾画差异，但可以得出结论，几乎所有的 OAR 都可以在 CT 或 MR 模式下以相似的变异性进行勾画，这有利于从仅 MR 和 MR 引导 RT 的角度来看待 MR 图像。
