Mid North Coast Cancer Institute Coffs Harbour, Mid North Local Health District, Coffs Harbour Health Campus, Coffs Harbour, NSW, 2450, Australia.
Western Cancer Centre Dubbo, Dubbo Base Hospital, Western NSW Local Health District, Dubbo, NSW, 2830, Australia.
Phys Eng Sci Med. 2023 Dec;46(4):1387-1397. doi: 10.1007/s13246-023-01306-8. Epub 2023 Sep 21.
The relative electron density (RED) parameter is ubiquitous throughout radiotherapy for clinical dosimetry and treatment planning purposes as it provides a more accurate description of the relevant radiological properties over mass density alone. RED is in practice determined indirectly from calibrated CT Hounsfield Units (HU). While CT images provide useful 3D information, the spectral differences between CT and clinical LINAC beams may impact the validity of the CT-ED calibration, especially in the context of novel tissue-mimicking materials where deviations from biologically typical atomic number to atomic weight ratios 〈Z/A〉 occur and/or high-Z materials are present. A theoretical basis for determining material properties directly in a clinical beam spectrum via an electron-density equivalent pathlength (eEPL) method has been previously established. An experimental implementation of this approach is introduced whereby material-specific measured percentage depth dose curves (PDDs) are regressed to a PDD measured in a reference material (water), providing an inference of 〈Z/A〉, which when combined with the physical density provides a determination of RED. This method is validated over a range of tissue-mimicking materials and compared against the standard CT output, as well as compositional information obtained from the manufacturer's specifications. The measured PDD regression method shows consistent results against both manufacturer-provided and CT-derived values between 0.9 and 1.15 RED. Outside of this soft-tissue range a trend was observed whereby the 〈Z/A〉 determined becomes unrealistic indicating the method is no longer reporting RED alone and the assumptions around the eEPL model are constrained. Within the soft-tissue RED range of validity, the regression method provides a practical and robust characterisation for unknown materials in the clinical setting and may be used to improve on the CT derived RED where high Z material components are suspected.
相对电子密度（RED）参数在放射治疗的临床剂量学和治疗计划中无处不在，因为它提供了比仅基于质量密度更准确的相关放射学特性描述。RED 实际上是通过校准 CT 亨氏单位（HU）间接确定的。虽然 CT 图像提供了有用的 3D 信息，但 CT 和临床 LINAC 光束之间的光谱差异可能会影响 CT-ED 校准的有效性，特别是在新型组织模拟材料的情况下，这些材料的原子数与原子量比（Z/A）偏离生物学典型值，或者存在高 Z 材料。以前已经建立了通过电子密度等效路径长度（eEPL）方法直接在临床光束谱中确定材料特性的理论基础。引入了这种方法的实验实现，其中特定材料的测量百分深度剂量曲线（PDD）回归到在参考材料（水）中测量的 PDD，提供了〈Z/A〉的推断，当与物理密度结合时，提供了 RED 的确定。该方法在一系列组织模拟材料上进行了验证，并与标准 CT 输出以及从制造商规格获得的成分信息进行了比较。测量的 PDD 回归方法与制造商提供的和 CT 衍生的值之间的一致性结果在 0.9 到 1.15 RED 之间。在软组织范围之外，观察到一个趋势，即确定的〈Z/A〉变得不现实，表明该方法不再单独报告 RED，并且 eEPL 模型的假设受到限制。在软组织 RED 的有效范围内，回归方法为临床环境中的未知材料提供了一种实用且稳健的特征描述，并且可以用于改进高 Z 材料成分可疑的 CT 衍生 RED。
