Department of Radiation Oncology, University of Würzburg, Josef-Schneider-Str. 11, 97080, Würzburg, Germany.
Med Phys. 2019 Feb;46(2):419-425. doi: 10.1002/mp.13308. Epub 2018 Dec 20.
To characterize the static properties of the anisotropy of dose contributions for different treatment techniques on real patient data (prostate cases). From this, we aim to define a class of treatment techniques with invariant anisotropy distribution carrying information of target coverage and organ-at-risk (OAR) sparing. The anisotropy presumably is a helpful quantity for plan adaptation problems.
The anisotropy field is analyzed for different intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) techniques for a total of ten planning CTs of prostate cases. Primary irradiation directions ranged from 5 to 15. The uniqueness of anisotropy was explored: In particular, the anisotropy distribution inside the planning treatment volume (PTV) and in its vicinity was investigated. Furthermore, deviations of the anisotropy under beam rotations were explored by direct plan comparison as an indicating the susceptibility of each planned technique to changes in the geometric plan configuration. In addition, plan comparisons enabled the categorization of treatment techniques in terms of their anisotropy distribution.
The anisotropy profile inside the PTV and in the transition between OAR and PTV is independent of the treatment technique as long as a sufficient number of beams contribute to the dose distribution. Techniques with multiple beams constitute a class of almost identical and technique-independent anisotropy distribution. For this class of techniques, substructures of the anisotropy are particularly pronounced in the PTV, thus offering good options for applying adaptation rules. Additionally, the techniques forming the mentioned class fortunately allow a better OAR sparing at constant PTV coverage. Besides the characterization of the distribution, a pairwise plan comparison reveals each technique's susceptibility to deviations which decreases for an increasing number of primary irradiation directions.
Techniques using many irradiation directions form a class of almost identical anisotropy distributions which are assumed to provide a basis for improved adaptation procedures. Encouragingly, these techniques deliver quite invariant anisotropy distributions with respect to rotations correlated with good plan qualities than techniques using few gantry angles. The following will be the next steps toward anisotropy-based adaptation: first, the quantification of anisotropy regarding organ deformations; and second, establishing the interrelation between the anisotropy and beam shaping.
在真实患者数据（前列腺病例）上，对不同治疗技术的剂量贡献各向异性的静态特性进行特征描述。由此，我们旨在定义一类具有各向异性分布不变性的治疗技术，这些技术携带目标覆盖和危及器官（OAR）保护的信息。各向异性推测是计划适应问题的一个有用的数量。
分析了总共 10 个前列腺病例的计划 CT 上不同强度调制放疗（IMRT）和容积调制弧形治疗（VMAT）技术的各向异性场。原发照射方向范围从 5 到 15。探索了各向异性的独特性：特别是，研究了计划治疗体积（PTV）内部及其附近的各向异性分布。此外，通过直接计划比较探索了各向异性在射束旋转下的偏差，作为每种计划技术对几何计划配置变化的敏感性的指示。此外，计划比较能够根据各向异性分布对治疗技术进行分类。
只要足够数量的射束贡献剂量分布，PTV 内和 OAR 与 PTV 之间的各向异性分布与治疗技术无关。具有多束的技术构成了一类几乎相同且与技术无关的各向异性分布。对于这个类别的技术，在 PTV 中，各向异性的子结构特别明显，因此为应用适应规则提供了良好的选择。此外，形成上述类别的技术幸运地允许在恒定 PTV 覆盖的情况下更好地保护 OAR。除了分布的特征化之外，成对的计划比较揭示了每种技术对偏差的敏感性，随着原发照射方向数量的增加而降低。
使用多个照射方向的技术形成了一类几乎相同的各向异性分布的类别，这些分布被认为为改进的适应过程提供了基础。令人鼓舞的是，这些技术提供了与使用少量龙门角度的技术相比具有更好的各向异性分布的不变性，并且具有相当的计划质量。基于各向异性的适应的下一步将是：首先，量化各向异性与器官变形的关系；其次，建立各向异性与射束成型之间的相互关系。
