Effects of Hounsfield number conversion on CT based proton Monte Carlo dose calculations
Effects of Hounsfield number conversion on CT based proton Monte Carlo dose calculations
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DOI：
10.1118/1.2715481
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发表时间：
2007-04-01
期刊：
影响因子：
3.8
通讯作者：
Paganetti, Harald
中科院分区：
文献类型：
--
作者：
Jiang, Hongyu;Seco, Joao;Paganetti, Harald
蒙特卡罗方法可针对患者计算机断层扫描 (CT) 几何形状提供最准确的剂量计算。准确性的提高至少部分是由于蒙特卡罗方法不是像分析算法中那样将人体组织视为不同密度的水，而是允许人体组织通过元素组成和质量密度来表征，从而可以准确考虑所有相关的电磁和核相互作用。另一方面，将 CT 亨斯菲尔德数转换为组织材料以进行蒙特卡罗剂量计算的算法引入了不确定性。亨斯菲尔德数和组织材料之间不存在简单的一一对应关系。为了研究亨斯菲尔德数转换对质子蒙特卡罗剂量计算的影响，使用 Geant4 蒙特卡罗代码模拟临床质子治疗计划。研究了三种亨斯菲尔德数到材料的转换方法。结果以大体肿瘤体积和临床靶体积的剂量体积直方图的形式进行比较。发现的差异通常很小，但在剂量测定上可能很显着。此外，不同的方法可能导致预测的质子束范围的偏差，特别是对于深质子场。通常，质量密度分配的轻微差异在目标区域中仅起很小的作用，而更显着的影响是由元素成分的不同分配引起的。在存在较大组织不均匀性的情况下，对于头颈部治疗，由于预测的肿瘤覆盖范围存在偏差，治疗计划决策可能会受到这些差异的影响。在目标区域之外，元素组成和质量密度分配的差异都可能发挥作用。这可能会对处于危险中的器官产生明显的影响，特别是在分散的布拉格峰半影或远端区域。此外，元素成分效应（水剂量与组织剂量）的重要性取决于组织类型，并且还受到核反应的影响。 (c) 2007 年美国医学物理学家协会。
The Monte Carlo method provides the most accurate dose calculations on a patient computed tomography (CT) geometry. The increase in accuracy is, at least in part, due to the fact that instead of treating human tissues as water of various densities as in analytical algorithms, the Monte Carlo method allows human tissues to be characterized by elemental composition and mass density, and hence allows the accurate consideration of all relevant electromagnetic and nuclear interactions. On the other hand, the algorithm to convert CT Hounsfield numbers to tissue materials for Monte Carlo dose calculation introduces uncertainties. There is not a simple one to one correspondence between Hounsfield numbers and tissue materials. To investigate the effects of Hounsfield number conversion for proton Monte Carlo dose calculations, clinical proton treatment plans were simulated using the Geant4 Monte Carlo code. Three Hounsfield number to material conversion methods were studied. The results were compared in forms of dose volume histograms of gross tumor volume and clinical target volume. The differences found are generally small but can be dosimetrically significant. Further, different methods may cause deviations in the predicted proton beam range in particular for deep proton fields. Typically, slight discrepancies in mass density assignments play only a minor role in the target region, whereas more significant effects are caused by different assignments in elemental compositions. In the presence of large tissue inhomogeneities, for head and neck treatments, treatment planning decisions could be affected by these differences because of deviations in the predicted tumor coverage. Outside the target area, differences in elemental composition and mass density assignments both may play a role. This can lead to pronounced effects for organs at risk, in particular in the spread-out Bragg peak penumbra or distall regions. In addition, the significance of the elemental composition effect (dose to water vs. dose to tissue) is tissue-type dependent and is also affected by nuclear reactions. (c) 2007 American Association of Physicists in Medicine.