Energy Dependence of Measured CT Numbers on Substituted Materials Used for CT Number Calibration of Radiotherapy Treatment Planning Systems

IntroductionFor accurate dose calculations, it is necessary to provide a correct relationship between the CT numbers and electron density in radiotherapy treatment planning systems (TPSs). The purpose of this study was to investigate the energy dependence of measured CT numbers on substituted materials used for CT number calibration of radiotherapy TPSs and the resulting errors in the treatment planning calculation doses.Materials and MethodsIn this study, we designed a cylindrical water phantom with different materials used as tissue equivalent materials for the simulation of tissues and obtaining the related CT numbers. For evaluating the effect of CT number variations of substituted materials due to energy changing of scanner (kVp) on the dose calculation of TPS, the slices of the scanned phantom at three kVp's were imported into the desired TPSs (MIRS and CorePLAN). Dose calculations were performed on two TPSs.ResultsThe mean absolute percentage differences between the CT numbers of CT scanner and two treatment planning systems for all the samples were 3.22%±2.57% for CorePLAN and 2.88%±2.11% for MIRS. It was also found that the maximum absolute percentage difference between all of the calculated doses from each photon beam of linac (6 and 15 MV) at three kVp's was less than 1.2%.DiscussionThe present study revealed that, for the materials with effective low atomic number, the mean CT number increased with increasing energy, which was opposite for the materials with an effective high atomic number. We concluded that the tissue substitute materials had a different behavior in the energy ranges from 80 to 130 kVp. So, it is necessary to consider the energy dependence of the substitute materials used for the measurement or calibration of CT number for radiotherapy treatment planning systems.引言：在放射治疗计划系统（TPS）中，要实现精准的剂量计算，必须建立CT值（CT numbers）与电子密度之间的正确对应关系。本研究旨在探究放射治疗TPS的CT值校准用替代材料的实测CT值的能量依赖性，以及由此引发的治疗计划剂量计算误差。 材料与方法：本研究设计了一款圆柱形水模体，采用多种材料作为组织等效材料，用于模拟人体组织并获取对应的CT值。为评估扫描仪管电压（kVp）变化导致的替代材料CT值波动对TPS剂量计算的影响，我们将三种管电压下扫描得到的模体断层影像导入至目标TPS（MIRS和CorePLAN），并在这两套TPS中完成剂量计算。 结果：所有样本中，CT扫描仪与两套治疗计划系统的CT值平均绝对百分比误差分别为CorePLAN的3.22%±2.57%，以及MIRS的2.88%±2.11%。此外，三种管电压下，直线加速器（linac）6 MV与15 MV光子束对应的所有计算剂量间的最大绝对百分比误差均小于1.2%。 讨论：本研究发现，有效原子序数较低的材料，其平均CT值随能量升高而上升；而有效原子序数较高的材料则呈现相反趋势。我们得出结论：在80~130 kVp的能量范围内，组织替代材料表现出不同的能量依赖性。因此，在放射治疗TPS的CT值测量或校准中，必须考虑替代材料的能量依赖性。