EPID在VMAT技术验收测试中的应用研究

Commission of volumetric modulated arc therapy with electronic portal image device

目的采用EPID实现直线加速器VMAT技术的验收研究。方法采用Shaper及EclipseTPS编辑并利用EPID对TrueBeam加速器VMAT技术中MLC到位精度、临床剂量等核心内容进行测试。测试例：TA：机架在0°、90°、270°时分别曝光0．5cm×20．0cm狭缝野；TB：不同剂量率及机架角速度组合时射野平坦度稳定性；TC：静态及旋转状态下MLC到位精度；TD：静态及旋转状态下MLC变速控制准确性；TE：加速器同时变剂量率及机架旋转速度的准确性；TF：临床病例。利用Matlab对测试结果进行分析。结果0°、90°、270°测得狭缝叠加后半高宽与零度时半高宽差别均为0．39mm。TB结果显示射野平坦度差别〈0．5％。TC测试显示EPID测得的狭缝中心与TPS设定的中心最大偏差为0．45mm。TD测试显示MLC位置实测值与TPS设定值差别最大为0．69mm。TE结果显示各狭缝相互间差别最大为0．42mm。临床病例以3％／3mm标准评价时γ通过率最低为96．4％。结论利用EPID能准确、便捷的完成VMAT技术验收,此方法为减轻物理师工作负担提供了一个良好选择。

Objective To apply electronic portal imaging device （EPID） in the quality assurance of volumetric modulated arc therapy （VMAT） delivered by TrueBeam linear accelerator. Methods The Shaper software and Eclipse treatment planning system （TPS） were used to edit the VMAT plan delivered by TrueBeam linear accelerator. The EPID was used to evaluate the setup accuracy of muhi-leaf collimator （MLC） and clinical dosimetry. TA ： a field with a size of 0. 5 cm×20.0 cm was exposed with gantry angles of 0°, 90°, and 270°, respectively; TB： the flatness and stability of the field were tested with different eombinations of dose rate and gantry angle speed ; TC ： the setup accuracy of MLC was evaluated with or without rotation ;TD：the accuracy of speed change control of MLC was evaluated with or without rotation ;TE： the accuracy of simultaneous changes in dose rate and gantry angle speed was evaluated;TF：Clinical patients were evaluated. The Matlab software was used to analyze the results. Results TA：the full width at half maximum of superimposed fields exposed with gantry angles of 0°, 90°, and 270°was 0. 39 mm larger than that of the field exposed with a gantry angle of 0% TB ：the difference in flatness was less than 0. 5% between fields with different combinations of dose rate and gantry angle speed. TC：the maximum difference in field center was 0. 45 mm between EPID measurement and TPS setting. TD：the maximum difference in MLC position was 0. 69 mm between EPID measurement and TPS setting. TE ： the maximum difference was 0. 42 mm between fields. TF ：for all the patients, the lowest γ passing rate was 96. 4% according to the criterion of 3%/3 mm. Conclusions EPID is an accurate and convenient approach for VMAT quality assurance, which provides an option for physicists to reduce workload.