While modern radiotherapy treatments can deliver a localized radiation dose to the tumor, healthy tissues at distance are inevitably exposed to scatter radiation that has been linked to late health effects such as second cancers. Quantifying the radiation dose received by tissues beyond the target is critical for research on such late health effects. However, the typical radiotherapy planning CT only covers part of the body near the target and the organs of interest for late effects research are not always included. Therefore, the purpose of this study was to develop a method for extending a partial-body pediatric CT scan for estimating organ doses beyond the original CT scan range. Our method uses a library of CT images for 359 pediatric patients from which a candidate patient is selected for providing surrogate anatomy. The most appropriate surrogate patient images to use for the extension are determined based on patient demographic information pulled from the image metadata. Image registration is performed through comparison of the patients' skeletons. The images showing closest similarity are adapted by a transformation method and appended to the original partial-body CT and a new structure file containing organ contours is written; we refer to this extended CT scan with organ contours as the Anatomical Predictive Extension (APE). To test the APE method, three patients with nearly full-body anatomy were extracted from the library, and a continuous subset of the images was removed to simulate a partial-body CT. The APE method was then applied to the partial-body CT to create extended anatomies, with the original images serving as ground truth. Radiotherapy plans were simulated using the Monte Carlo code XVMC on both the original and APE anatomies, with the original serving as ground truth. Three pediatric radiotherapy cases were considered for performance testing: (1) head CT for a simulated brain tumor extended to chest; (2) superior chest CT for simulated Hodgkin's lymphoma extended to inferior chest; (3) pelvic CT for Wilms tumor extended to superior chest. Three geometric metrics (Dice similarity coefficient, overlap fraction, and volume similarity) were calculated to quantify the differences between the original patient and the extended anatomies. In all cases, calculated organ doses showed good agreement between the original and APE anatomies. The average absolute relative dose difference across all organs considered for the three cases was 11%, 12% and 15%, respectively. The APE method is useful for estimating radiation doses to peripheral organs in support of research on late effects following radiotherapy.

中文翻译：

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儿科部分 CT 在放疗患者全身器官剂量估计中的现实扩展


虽然现代放射治疗可以向肿瘤提供局部辐射剂量，但远处的健康组织不可避免地会暴露在散射辐射下，这些辐射与晚期健康影响（如二次癌症）有关。量化目标以外组织接收到的辐射剂量对于研究这种晚期健康影响至关重要。然而，典型的放疗计划 CT 仅覆盖目标附近的身体部位，并且并不总是包括用于迟发效应研究的感兴趣器官。因此，本研究的目的是开发一种方法，将部分身体儿科 CT 扫描扩展到原始 CT 扫描范围之外，以估计器官剂量。我们的方法使用 359 名儿科患者的 CT 图像库，从中选择一名候选患者来提供替代解剖结构。最适合用于扩展的代理患者图像是根据从图像元数据中提取的患者人口统计信息确定的。通过比较患者的骨骼进行图像配准。显示最相似的图像通过转换方法进行调整并附加到原始部分体 CT 上，并写入包含器官轮廓的新结构文件;我们将这种带有器官轮廓的扩展 CT 扫描称为解剖学预测扩展 （APE）。为了测试 APE 方法，从库中提取了 3 名具有近全身解剖结构的患者，并删除了图像的连续子集以模拟部分体 CT。然后将 APE 方法应用于部分体 CT 以创建扩展的解剖结构，以原始图像作为基本实况。 在原始和 APE 解剖结构上使用蒙特卡洛代码 XVMC 模拟放疗计划，以原始图像作为基本实况。考虑对 3 例儿科放疗病例进行性能测试：（1） 针对延伸到胸部的模拟脑肿瘤的头部 CT;（2） 模拟霍奇金淋巴瘤的胸部上部 CT 延伸到下胸部;（3） 肾母细胞瘤的盆腔 CT 延伸到上胸部。计算三个几何指标 （Dice 相似系数、重叠分数和体积相似性） 以量化原始患者和扩展解剖结构之间的差异。在所有情况下，计算的器官剂量都显示原始解剖和 APE 解剖之间具有良好的一致性。这三种病例所考虑的所有器官的平均绝对相对剂量差异分别为 11% 、 12% 和 15%。APE 方法可用于估计外周器官的辐射剂量，以支持放疗后迟发效应的研究。