用锥形束CT技术测量热塑成型膜固定患者的放疗摆位误差

Cone beam CT in analysis of set-up errors with thermoplastic device immobilization

目的测量热塑成型膜固定患者放疗的摆位误差，确定由临床靶区（CTV）或内靶区（ITV）外放产生计划靶区（PTV）的间距。方法接受图像引导放疗的患者120例，其中头颈部肿瘤13例（109组图像数据），胸部肿瘤67例（279组图像数据），腹部肿瘤40例（171组图像数据）。所有患者均采用热塑成型体罩或面罩做体位固定。在分次治疗前采用锥形束CT技术获取三维CT图像，把此图像和计划设计所用的螺旋CT图像进行配准，得到摆位误差数值。因未做在线校位，该误差值代表常规摆位方法的摆位误差。摆位误差由X（左右）、Y（头脚）、Z（腹背）方向的平移分量和绕3个轴线的旋转角度分量构成。统计分析摆位误差数据，用二参数法计算产生PTV的间距。结果头颈部肿瘤3个方向的平移误差分别为（0．13±0．15）、（0.13±0．17）、（0．11±0．14）cm，旋转角度误差分别为1．05°±0．77°、0．87°±1．13°和0．68°±0．89°，外放间距分别为0．37、0．38、0.31cm。胸部肿瘤的平移误差分别为（0．20±0．27）、（0.34±0.44）、（0．25±0．31）cm；旋转角度误差分别是1．06°±1．45°、0．85°±1.23°和0.78°±1．08°；外放间距分别是0.59、1．00、0．72cm。腹部肿瘤3个方向的平移误差分别为（0．23±0．30）、（0．37±0．45）cm和（0．27±0．34）cm，旋转角度误差分别为1．22°±1．56°、1．05°±1．44°、0.98°±1.24°，外放间距分别为0．66、1．05、0．78cm。结论应用锥形束CT技术可准确测量常规摆位方法的摆位误差，继而确定靶区外放间距。若考虑靶区在1个分次内的运动，则胸腹部肿瘤应在ITV基础上应用上述间距产生PTV。

Objective To evaluate the systematic and random set-up errors in patients immobilized with thermoplastic device during radiotherapy, and to determine the proper margins extended from clinical target volume（ CTV ） or internal target volume （ ITV ） to planning target volume （ PTV ）. Methods From March 2007 to September 2007,120 patients were included in this study, including 13 receiving head and neck irradiation,67 thoracic irradiation and 40 abdominal irradiation. All patients were immobilized with thermoplastic device and received CT simulation and intensity modulated radiation therapy（IMRT）. X-ray cone beam CT was regularly performed before treatment and the images were compared with the simulation CT images. The shift and rotation in right-left（R-L） ,superior-inferior（S-I） and anterior-posterior（A-P） directions were recorded and analyzed. The shift margin from CTV or ITV to PTV was calculated with the equation, margin = 2＇mean ± 0.7＇standard deviation. Results In head and neck region,the shift errors in R-L,S-I and A-P directions were（0.13 ±0.15） cm,（0.13 ±0.17） cm and（0.11 ±0.14） cm,and the corresponding rotation errors were 1.05° ±0.77° ,0. 87° ± 1.13° and 0. 68°±0.89°. The margins from CTV to PTV were 0.37 cm,0.38 cm and 0.31 cm,respectively. In thoraci region,the shift errors in R-L,S-I and A- P directions were （0.20 ± 0.27 ） cm, （0.34 ± 0.44） cm and （0.25± 0.31 ） cm, and the corresponding rotation errors were 1.06°± 1.45° ,0.85°± 1.23° and 0.78°± 1.08° The shift margins from ITV to PTV were 0.59 cm, 1.00 cm and 0.72 cm. In abdominal region, the shift errors in R-L, S-I and A-P directions were （0.23±0.30） cm, （0.37 ± 0.45 ） cm and （ 0.27 ± 0.34） cm, and the corresponding rotation errors were 1.22°± 1.56°, 1.05°± 1.44°and 0.98°± 1.24% The shift margins from CTV or ITV to PTV were 0.66 cm, 1.05 cm and 0.78 cm. Conclusions Cone beam CT can be used in the precise measure of set-up errors,which can provide institution-specific margins for PTV designing in patients immobilized with thermoplastic device.