用锥形束CT技术测量三维后装与外照射调强融合放疗中施源器位置及外照射摆位误差

Measuring the Applicator Position and the Interfraction Setting-up Errors by Cone Beam CT in Brachytherapy combined Intensity-modulated Radiation Therapy

目的:测量施源器位置及外照射摆位的误差,评估自行设计的后装CT定位治疗床对施源器的固定效果;确定外照射调强治疗靶区外放产生计划靶区(PTV)的间距。方法:接受三维后装与外照射调强融合放疗的患者50人次,每位患者在每次布放施源器前均采用热塑成型膜固定在自行设计的后装CT定位治疗床上,而后植入并固定施源器,作定位螺旋CT扫描。在每次治疗前进行锥形束CT扫描获取三维CT图像,将此图像和计划设计所用的螺旋CT图像进行配准,得到施源器位置及外照射摆位误差的数值。误差由X(左右)、Y(头脚)、Z(腹背)方向的平移分量和围绕3个轴线的旋转角度分量(Rx、Ry、Rz)构成。统计分析误差数据,用二参数法计算产生计划靶区的间距。结果:灰度配准的平移误差为X(0.18±0.15)cm、Y(0.22±0.12)cm、Z(0.17±0.13)cm,旋转角度误差为RX(1.71°±1.09°)、RY(1.31°±1.04°)、RZ(0.73°±0.44°);施源器作为配准标志手动配准的平移误差为X(0.11±0.09)cm、Y(0.12±0.05)cm、Z(0.16±0.10)cm,旋转角度误差为RX(0.51°±0.16°)、RY(0.15°±0.08°)、RZ(0.23°±0.21°);计划靶区的X、Y、Z外放间距分别为0.47、0.58、0.43cm。结论:应用锥形束CT技术可准确测量自行设计的后装定位治疗床对施源器的固定摆位误差,确定产生计划靶区所需的外放间距能有效地保证三维后装放疗的精确实施;

Objective： To estimate the fixing effect of brachytherapy bed on applicator by measuring the applicator position and the interfraction setting-up errors and determine the proper margins which extends from clinical target volume（CTV） to planning target volume（PTV）. Methods： Cervical cancer patients were treated with intensity-modulated radiation therapy （IMRT）combined with brachytherapy fifty times . After fixed with thermoplastic device immobilization on the treatment bed, applicator was installed into the vaginal and uterine of the patients and CT scan was taken to patients. We matched the daily image of the cone beam CT with the plan- ning target CT and acquired the error data of daily set-up. Statistical analysis of error data, including variables of X, Y, Z directions and the three axis rotation angle component could be done with the two parameters methods. Results： The shift error of the intensity registration was （0. 18 ± 0. 15） cm for X, （0. 22± 0. 12） cm for Y, （0. 17 ± 0. 13 ） cm for Z ; （ 1.71 °± 1.09° ） for rotation angle （RX）, （1.31±1.04°） for RY, （0. 73° ± 0. 44°） for RZ. Using the manual registration with the applicator, the shift error was （0. 11 ±0.09） cm for X,（0. 12±0.05）cm for Y, （0. 16 °±0. 10 °） cm for Z; rotation angle error was （0.51°± 0. 16°） for RX, （0. 15° ±0.08°） for RY, （0.23±0.21°） for RZ. The margin of the plan target volume of X, Y, Z were 0.47cm, 0.58cm,0.43cm, respectively. Conclusion： Applicator can be fixed firmly by brachytherapy bed, which can make assure accurate brachytherapy. And cone beam CT can be used to easure set-up errors precisely, which can provide institution-specific margins for PTV.