Grantham Kevin K, Li Hua, Zhao Tianyu, Klein Eric E
University of Missouri.
J Appl Clin Med Phys. 2015 Nov 8;16(6):100-109. doi: 10.1120/jacmp.v16i6.5516.
The purpose of this study was to investigate the impact of tube potential (kVp) on the CT number (HU) to proton stopping power ratio (PSPR) conversion. The range and dosimetric change introduced by a mismatch in kVp used for the CT scan and the HU to PSPR table, based on a specific kVp, used to calculate dose are analyzed. Three HU to PSPR curves, corresponding to three kVp settings on the CT scanner, were created. A treatment plan was created for a single beam in a water phantom passing through a wedge-shaped bone heterogeneity. The dose was recalculated by changing only the HU to PSPR table used in the dose calculation. The change in the position of the distal 90% isodose line was recorded as a function of heterogeneity thickness along the beam path. The dosimetric impact of a mismatch in kVp between the CT and the HU to PSPR table was investigated by repeating this procedure for five clinical plans comparing DVH data and dose difference distributions. The HU to PSPR tables diverge for CT numbers greater than 200 HU. In the phantom plan, the divergence of the tables resulted in a difference in range of 1.6 mm per cm of bone in the beam path, for the HU used. For the clinical plans, the dosimetric effect of a kVp mismatch depends on the amount of bone in the beam path and the proximity of OARs to the distal range of the planned beams. A mismatch in kVp between the CT and the HU to PSPR table can introduce inaccuracy in the proton beam range. For dense bone, the measured range difference was approximately 1.6 mm per cm of bone along the beam path. However, the clinical cases analyzed showed a range change of 1 mm or less. Caution is merited when such a mismatch may occur.
本研究的目的是调查管电压（kVp）对CT值（HU）与质子阻止本领比（PSPR）转换的影响。分析了CT扫描所使用的kVp与基于特定kVp用于计算剂量的HU至PSPR表不匹配所引入的射程和剂量学变化。创建了对应于CT扫描仪上三个kVp设置的三条HU至PSPR曲线。针对水模体中穿过楔形骨不均匀性的单束射野创建了一个治疗计划。仅通过改变剂量计算中使用的HU至PSPR表来重新计算剂量。记录沿射野路径远端90%等剂量线位置的变化作为不均匀性厚度的函数。通过对五个临床计划重复此过程并比较DVH数据和剂量差异分布，研究了CT与HU至PSPR表之间kVp不匹配的剂量学影响。对于大于200 HU的CT值，HU至PSPR表会出现偏差。在模体计划中，对于所使用的HU，表的偏差导致射野路径中每厘米骨的射程差异为1.6 mm。对于临床计划，kVp不匹配的剂量学效应取决于射野路径中的骨量以及危及器官（OAR）与计划射野远端射程的接近程度。CT与HU至PSPR表之间的kVp不匹配会在质子束射程中引入误差。对于致密骨，沿射野路径测量的射程差异约为每厘米骨1.6 mm。然而，所分析的临床病例显示射程变化为1 mm或更小。当可能出现这种不匹配时应谨慎。
