Wohlfahrt Patrick, Möhler Christian, Richter Christian, Greilich Steffen
OncoRay - National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Helmholtz-Zentrum Dresden - Rossendorf, Dresden, Germany; Helmholtz-Zentrum Dresden - Rossendorf, Institute of Radiooncology - OncoRay, Dresden, Germany.
German Cancer Research Center (DKFZ), Heidelberg, Germany; National Center for Radiation Research in Oncology (NCRO), Heidelberg Institute for Radiation Oncology (HIRO), Heidelberg, Germany.
Int J Radiat Oncol Biol Phys. 2018 Jan 1;100(1):244-253. doi: 10.1016/j.ijrobp.2017.09.025. Epub 2017 Sep 18.
To determine the accuracy of particle range prediction for proton and heavier ion radiation therapy based on dual-energy computed tomography (DECT) in a realistic inhomogeneous geometry and to compare it with the state-of-the-art clinical approach.
A 3-dimensional ground-truth map of stopping-power ratios (SPRs) was created for an anthropomorphic head phantom by assigning measured SPR values to segmented structures in a high-resolution CT scan. This reference map was validated independently comparing proton transmission measurements with Monte Carlo transport simulations. Two DECT-based methods for direct SPR prediction via the Bethe formula (DirectSPR) and 2 established approaches based on Hounsfield look-up tables (HLUTs) were chosen for evaluation. The SPR predictions from the 4 investigated methods were compared with the reference, using material-specific voxel statistics and 2-dimensional gamma analysis. Furthermore, range deviations were analyzed in an exemplary proton treatment plan.
The established reference SPR map was successfully validated for the discrimination of SPR and range differences well below 0.3% and 1 mm, respectively, even in complex inhomogeneous settings. For the phantom materials of larger volume (mainly brain, soft tissue), the investigated methods were overall able to predict SPR within 1% median deviation. The DirectSPR methods generally performed better than the HLUT approaches. For smaller phantom parts (such as cortical bone, air cavities), all methods were affected by image smoothing, leading to considerable SPR under- or overestimation. This effect was superimposed on the general SPR prediction accuracy in the exemplary treatment plan.
DirectSPR predictions proved to be more robust, with high accuracy in particular for larger volumes. In contrast, HLUT approaches exhibited a fortuitous component. The evaluation of accuracy in a realistic phantom with validated ground-truth SPR represents a crucial step toward possible clinical application of DECT-based SPR prediction methods.
在真实的非均匀几何结构中，确定基于双能计算机断层扫描（DECT）的质子和重离子放射治疗中粒子射程预测的准确性，并将其与当前的临床先进方法进行比较。
通过将测量的阻止本领比（SPR）值分配给高分辨率CT扫描中的分割结构，为人体头部模型创建了一个三维真实的SPR地图。通过将质子传输测量结果与蒙特卡罗传输模拟进行独立比较，对该参考地图进行了验证。选择了两种基于DECT通过贝特公式直接预测SPR的方法（DirectSPR）和两种基于亨氏单位查找表（HLUT）的既定方法进行评估。使用特定材料的体素统计和二维伽马分析，将这四种研究方法的SPR预测结果与参考结果进行比较。此外，在一个示例性质子治疗计划中分析了射程偏差。
即使在复杂的非均匀环境中，所建立的参考SPR地图也成功验证了SPR和射程差异的辨别能力，分别远低于0.3%和1毫米。对于较大体积的模型材料（主要是脑、软组织），所研究的方法总体上能够在中位数偏差1%以内预测SPR。DirectSPR方法通常比HLUT方法表现更好。对于较小的模型部分（如皮质骨、气腔），所有方法都受到图像平滑的影响，导致SPR被显著低估或高估。这种影响叠加在示例性治疗计划中的一般SPR预测准确性上。
DirectSPR预测被证明更稳健，特别是对于较大体积具有高精度。相比之下，HLUT方法表现出一定的偶然性。在具有经过验证的真实SPR的模型中评估准确性，是基于DECT的SPR预测方法迈向可能临床应用的关键一步。
