Department of Accelerator and Medical Physics, National Institute of Radiological Sciences, Chiba, Japan.
Med Phys. 2012 Feb;39(2):1016-20. doi: 10.1118/1.3679339.
In treatment planning of charged-particle radiotherapy, patient heterogeneity is conventionally modeled as variable-density water converted from CT images to best reproduce the stopping power, which may lead to inaccuracies in the handling of multiple scattering and nuclear interactions. Although similar conversions can be defined for these individual interactions, they would be valid only for specific CT systems and would require additional tasks for clinical application. This study aims to improve the practicality of the interaction-specific heterogeneity correction.
The authors calculated the electron densities and effective densities for stopping power, multiple scattering, and nuclear interactions of protons and ions, using the standard elemental-composition data for body tissues to construct the invariant conversion functions. The authors also simulated a proton beam in a lung-like geometry and a carbon-ion beam in a prostate-like geometry to demonstrate the procedure and the effects of the interaction-specific heterogeneity correction.
Strong correlations were observed between the electron density and the respective effective densities, with which the authors formulated polyline conversion functions. Their effects amounted to 10% differences in multiple-scattering angle and nuclear interaction mean free path for bones compared to those in the conventional heterogeneity correction. Although their realistic effect on patient dose distributions would be generally small, it could be at the level of a few percent when a carbon-ion beam traverses a large bone.
The present conversion functions are invariant and may be incorporated in treatment planning systems with a common function relating CT number to electron density. This will enable improved beam dose calculation while minimizing initial setup and quality management of the user's specific system.
在带电粒子放射治疗的计划制定中，患者异质性通常被建模为从 CT 图像转换而来的可变密度水，以最佳地再现阻止本领，这可能导致多次散射和核相互作用的处理不准确。尽管可以为这些单独的相互作用定义类似的转换，但它们仅对特定的 CT 系统有效，并且需要额外的任务才能用于临床应用。本研究旨在提高特定相互作用异质性校正的实用性。
作者使用人体组织的标准元素组成数据计算了质子和离子的阻止本领、多次散射和核相互作用的电子密度和有效密度，以构建不变的转换函数。作者还模拟了肺部样几何形状中的质子束和前列腺样几何形状中的碳离子束，以演示该过程以及特定相互作用异质性校正的效果。
观察到电子密度与各自的有效密度之间存在很强的相关性，作者用这些密度来制定折线转换函数。与传统异质性校正相比，它们在骨骼中的多次散射角和核相互作用平均自由程方面的影响达到了 10%的差异。尽管它们对患者剂量分布的实际影响通常较小，但当碳离子束穿过大骨时，其影响可能达到百分之几。
本研究提出的转换函数是不变的，可以与将 CT 数与电子密度相关联的通用函数结合在治疗计划系统中使用。这将在最小化用户特定系统的初始设置和质量管理的同时，实现对光束剂量计算的改进。
