Hosier K, Wu C, Beck K, Radevic M, Asche D, Bareng J, Kroner A, Lehmann J, Logsdon M, Dutton S, Rosenthal S
Radiological Associates of Sacramento, Sacramento, CA.
Australian Clinical Dosimetry Service, Yallambie, AUS.
Med Phys. 2012 Jun;39(6Part12):3740-3741. doi: 10.1118/1.4735222.
To investigate various issues for clinical implementation of aSi EPID panels for IMRT/VMAT QA.
Six linacs are used in our clinic for EPID-based plan QA; two Varian Truebeams, two Varian 2100 series, two Elekta Infiniti series. Multiple corrections must be accounted for in the calibration of each panel for dosimetric use. Varian aSi panels are calibrated with standard dark field, flood field, and 40×40 diagonal profile for beam profile correction. Additional corrections to account for off-axis and support arm backscatter are needed for larger field sizes. Since Elekta iViewGT system does not export gantry angle with images, a third-party inclinometer must be physically mounted to back of linac gantry and synchronized with data acquisition via iViewGT PC clock. A T/2 offset correctly correlates image and gantry angle for arc plans due to iView image time stamp at the end of data acquisition for each image. For both Varian and Elekta panels, a 5 MU 10×10 calibration field is used to account for the nonlinear MU to dose response at higher energies. Acquired EPID images are deconvolved via a high pass filter in Fourier space and resultant fluence maps are used to reconstruct a 3D dose 'delivered' to patient using DosimetryCheck. Results are compared to patient 3D dose computed by TPS using a 3D-gamma analysis.
120 IMRT and 100 VMAT cases are reported. Two 3D gamma quantities (Gamma(V10) and Gamma(PTV)) are proposed for evaluating QA results. The Gamma(PTV) is sensitive to MLC offsets while Gamma(V10) is sensitive to gantry rotations. When a 3mm/3% criteria and 90% or higher 3D gamma pass rate is used, all IMRT and 90% of VMAT QA pass QA.
After appropriate calibration of aSi panels and setup of image acquisition systems, EPID based 3D dose reconstruction method is found clinically feasible.
研究用于调强放射治疗（IMRT）/容积调强弧形治疗（VMAT）质量保证（QA）的非晶硅电子射野影像装置（aSi EPID）面板临床应用中的各种问题。
我们科室使用六台直线加速器进行基于EPID的计划QA；两台瓦里安Truebeam直线加速器、两台瓦里安2100系列直线加速器、两台医科达Infiniti系列直线加速器。每个用于剂量测定的面板在校准时必须考虑多种校正。瓦里安aSi面板通过标准暗场、均整场和40×40对角线轮廓进行校准，以校正射野轮廓。对于更大的射野尺寸，需要进行额外的校正以考虑离轴和支撑臂散射。由于医科达iViewGT系统不会随图像导出机架角度，必须在直线加速器机架背面物理安装第三方倾角仪，并通过iViewGT PC时钟与数据采集同步。由于在每个图像数据采集结束时iView图像有时间戳，对于弧形计划，T/2偏移可正确关联图像和机架角度。对于瓦里安面板和医科达面板，均使用5MU的10×10校准野来考虑较高能量下MU与剂量响应的非线性关系。采集的EPID图像在傅里叶空间通过高通滤波器进行去卷积，所得注量图用于使用剂量验证软件（DosimetryCheck）重建“给予”患者的三维剂量。将结果与使用三维伽马分析由治疗计划系统（TPS）计算的患者三维剂量进行比较。
报告了120例IMRT和100例VMAT病例。提出了两个三维伽马量（Gamma(V10)和Gamma(PTV)）用于评估QA结果。Gamma(PTV)对多叶准直器（MLC）偏移敏感，而Gamma(V10)对机架旋转敏感。当使用3mm/3%标准和90%或更高的三维伽马通过率时，所有IMRT和90%的VMAT QA通过QA。
在对aSi面板进行适当校准和图像采集系统设置后，基于EPID的三维剂量重建方法在临床上是可行的。
