中文摘要：

目的 探讨电子射野影像装置（EPID）位置误差对容积旋转调强放疗（VMAT）三维剂量验证的影响.方法 5枚Suremark SL-20铅点固定于Elekta托盘上,通过采集机架在0～360°旋转中EPID图像,分析各角度下EPID相对于加速器机头的位置偏移,根据该偏移对进行三维剂量重建的EPID图像进行位置误差修正,分析EPID运动误差对剂量重建的影响.分别对16例鼻咽癌患者的VMAT计划的双弧、顺时针弧（弧1）、逆时针弧（弧2）的重建剂量与计划剂量做γ分析,并对修正前后的γ分析结果进行分析.结果 相对于0°,源到探测器距离（SID）在180°时误差最大,为1.20 cm.考虑SID变化后计算的EPID上下（y）方向误差最大为2.28 mm（等中心层面）,左右（x）方向误差在±0.5 mm以内.对16例鼻咽癌双弧VMAT治疗计划进行治疗前三维剂量验证,EPID y方向位置误差修正后3Dγ通过率明显提高,5％/3 mm标准下的γ通过率提高分别为双弧（4.12±1.67）％（t=-9.86,P＜0.05）,弧1（3.47 ±1.64）％（t=-8.46,P＜0.05）,弧2（5.08±1.30）％（t=-15.63,P＜0.05）;3％/3 mm标准下,γ通过率提高分别为双弧（7.63±2.24）％（t=-13.63,P ＜0.05）,弧1（6.03±2.07）％（t=-11.66,P＜0.05）,弧2（9.17±2.23）％（t=-16.41,P＜0.05）.y方向修正后,再进行x方向修正,5％/3 mm和3％/3 mm γ通过率的平均值分别提高0.23％和0.24％.结论 EPID沿加速器机架到治疗床方向运动误差明显,对三维剂量重建影响较大.在基于EPID的剂量重建中,应对其进行修正,以重建较准确的患者三维剂量分布.

英文摘要：

 Objective To analyze the impact of electronic portal imagingdevice （EPID） position error on three-dimensional dose verification of volumetric modulated arc therapy （VMAT）.Metbods Five Suremark SL-20 lead points were fixed on Elekta tray,and EPID images were collected in 0-360° rotation,one image per 5°.The position error relative to the accelerator was analyzed via Matlab.Then the images position error was corrected according to the analysis,and the 3D dose was reconstructed with the corrected images.The dose distributions of double arcs,clockwise arc（arc 1）,and counterclockwise arc （arc 2） of 16 nasopharyngeal carcinoma patients＇ VMAT plan were evaluated by γ analysis,and the results of before and after position error correction were compared.Results Compared to 0° gantry angle,the error of source to the image distance （SID） was maximum （1.20 cm） when the gantry angle was 180°.On account of the SID change,the maximum error along the up-down （y） direction in the iso-center planar was 2.28 mm and the left-right （x） direction error was within ± O.5 mm.The 3D γ analyses of 16 nasopharyngeal carcinoma in VMAT plans were obviously increased after the position error along y was corrected.The double arcs,arc1 and arc 2 were increased by （4.12 ±1.67） % （t =-9.86,P〈 0.05）,（3.47±1.64） % （t=-8.46,P〈 0.05） and （5.08±1.30） % （t=-15.63,P〈 0.05） in 5%/3 mm standard,respectively.However,in 3%/3 mm standard,γ value of the double arcs,arc 1 and arc2 were increased by （7.63 ±2.24） % （t =-13.63,P〈 0.05）,（6.03 ±2.07） % （t =-11.66,P〈 0.05）,（9.17 ±2.23） % （t =-16.41,P〈 0.05）,respectively.Since the EPID position error along x was corrected after y,the 3D γ analysis of reconstruction dose indicated that the average of the 5%/3 mm and 3%/3 mm γ value were increased by 0.23% and 0.24%,respectively.Conclusions EPID motion error along the gantry to table direction of the accelerator can＇t be ignored.When reconstruct dose base