千伏锥形束CT引导放疗系统最小误差与剩余误差分析

目的 分析千伏锥形束CT(kVCBCT)引导放疗系统可以发现的最小摆位误差和校正后剩余摆位误差。方法 采用小球(ball-bearing,BB)体模,比较经kVCBCT引导放疗系统得到的位移值与设定的位移值。设定的位移值分别为0.5、1.0、1.5和2.0 mm。侧向、纵向和垂直3个方向上的位移值均每日分析1次,共分析5次。校正后剩余摆位误差分析,采用CIRS Model 002LFC胸部体模,在侧向、纵向和垂直3个方向上均设置摆位误差,设置的数值为0、±5、±10和±15 mm。采用点标记法,共同配准体模的定位CT影像与kVCBCT影像,得出侧向、纵向和垂直3个方向上的摆位误差。根据配准结果移动治疗床,至侧向、纵向和垂直3个方向上摆位误差减少到最小。对校正位置后的体模再次行kVCBCT扫描,将再次获得的kVCBCT影像与定位CT采用点标记法配准,得到的侧向、纵向和垂直3个方向上的数值为经kVCBCT系统引导后剩余摆位误差。每日分析1次,共分析7次。结果 kVCBCT引导放疗系统可以发现的误差至少为0.5 mm。侧向、纵向和垂直3个方向上引导后剩余误差分别为(0.2±0.4)、(0.1±0.4)和(-0.4±0.3)mm,对3组数值行方差分析进行两两比较:侧向和纵向剩余误差无差别(P=0.63),垂直向和侧向剩余摆位误差的差异具有统计学意义(P=0.01),垂直向和纵向剩余摆位误差差异具有统计学意义(P=0.02)。结论 建立了kVCBCT引导放疗系统可以发现最小摆位误差和校正后剩余摆位误差的分析方法,kVCBCT引导放疗系统可以发现的误差至少为0.5 mm,侧向、纵向和垂直3个方向上引导后剩余误差约为1 mm。

Analysis of the minimal detected error and residual error for kilovoltage cone-beam CT guided radiotherapy system

Objective To assess the minimal detected error and residual error for the kilovoltage (kV) cone-beam CT guided radiotherapy system. Methods The ball-bearing phantom was used for analyzing the minimal detected error of the system. Different groups of real shift were intentionally set up and the true shift and the detected shift of the system were compared. The minimal real shift, which can be resolved by the system, is considered as the minimal detected error of the system. CIRS Model 002LFC thoracic phantom was used to analyze the residual error of the system. The intended set_up error was detected by the alignment of the kVCBCT and the planning CT. The phantom set_up error was then corrected by moving the couch of the accelerator accordingly. A second CBCT scan was acquired immediately after the correction to evaluate the residual set-up error.Results The minimal detected error of the system was 0.5 mm. The result of the phantom measurement performed within 2 weeks showed that the average residual errors (±SD) in the online image guided process were (0.2±0.4), (0.1±0.4) and (-0.4±0.3) mm in the lateral, longitudinal, and vertical directions, respectively.Conclusions This study developed the methods of analyzing the minimal detected error and residual error for kilovoltage cone-beam CT guided radiotherapy system. The minimal detected error of the system was 0.5 mm. The residual error of the system is about 1 mm.