中文摘要：

目的 研究不同类型电子射野影像装置（EPID）机械运动的相对位置偏差,为基于EPID进行调强放疗剂量验证提供位置误差的修正依据。方法 对3种不同类型EPID （瓦里安aS1000、aS500和医科达iViewGT）采用5 cm×5 cm射野进行照射。通过自编软件系统对EPID采集的射野图像进行分析,并确定射野中心点在EPID上投影。对臂架0°~360°射野中心点在EPID上投影位置进行分析,并与臂架0°时位置比较以确定相对位置偏移。结果 在瓦里安aS1000、aS500、医科达iViewGT的EPID最大相对偏移左右方向上分别为（－0.23±0.17）、（2.94±0.17）、（0.35±0.09） mm,上下方向上分别为（－4.16±0.20）、（－4.15±0.25）、（－1.66±0.11） mm。对上下方向上的相对位移采用四次方经验函数可较好地进行拟合。结论 不同类型EPID在不同角度下相对位置误差不同,且在上下方向上明显较左右方向大,应用EPID进行调强验证时必需考虑对在不同臂架角度下相对位置误差进行修正。

英文摘要：

Objective To study corrective method for displacement in the procedure of electronic portal imaging device （EPID）-based intensity-modulated radiotherapy dose valuation by studying the relative mechanical displacement of different vendor EPID （aS1000,Varian;aS500,Varian;iViewGT, Elekta）. Methods A 5 cm×5 cm field was set up to acquire portal images for three kinds of EPID, then a in house software was used to analysis the portal images. The relative displacement was acquired via analyzing a series of comparation between center positions of gantry angle ranging from 0° to 360° and gantry angle at 0°. Results In the lateral direction, the maximum relative displacement of EPID with aS1000,S500 and iViewGT were （-0.23±0.17） mm,（2.94±0.17） mm and （0.35±0.09） mm,respectively. In the longitude direction, the displacements were （-4.16±0.20） mm,（-4.15±0.25） mm and （-1.66±0.11） mm, respectively. As to longitude direction, the displacements could be well fitted with the usage of quadruplicate empiric function. Conclusions There is a significant difference at the aspect of relative displacement between different vendors EPID at different gantry angles. And the displacement in the longitude direction is obviously larger than in the lateral direction. The relative displacement should be corrected when applying EPID to the intensity-modulated radiotherapy dose verification at different gantry angles.