常规分割非均整模式部分弧旋转调强计划剂量验证的分析

Analysis on dosimetric verification of flattening filter free conventional segmentation partial arc RapidArc plans

对比了aSi-1200 EPID与Octavius 729探测器在常规分割非均整(Flattening filter free,FFF)部分弧RapidArc计划验证中的应用,探究两者的适用性和局限性及对MLC(Multi-leaf collimator)模型参数变化的γ响应。获取水模体不同大小方野的通量图,比较通过中心轴Crossline和Inline的CU(Calibration unit)值曲线评估aSi-1200 EPID背向散射屏蔽层的有效性。选取50个计划,在不同标准和阈值行2 Dγ指数通过率分析(全局归一),将γ值较低的计划数据导入Verisoft软件重建剂量。任选一计划,引入不同量值的MLC透射因子(Transmission factor,TF)和剂量叶片间隙(Dosimetric leaf gap,DLG)偏差,共创建80个修正剂量分布,将原始计划的测量剂量与修正计划的TPS剂量行γ通过率评估。结果表明:正的离轴位置跌落区CU值差异达2~3倍,Inline方向测量值整体偏小(Varian EDGE加速器),而Halcyon加速器两个方向CU值轮廓近乎完全重合。aSi-1200 EPID的γ通过率中值和均值均高于Octavius 729矩阵,基于Octavius 729和Octavius 1500矩阵测量重建的剂量与TPS对比部分剂量学偏差D D%:PGTV的D 98%(−9.60%,−14.30%),Heart的V_(30)(−6.80%,−2.36%),Lung L的V_(30)(−8.11%,15.79%),Lung R的V_(30)(100%,50%),Cord的D_(max)(3.55%,−0.82%)。随着TF和DLG偏离TPS预设值的增加,两种方式得到的γ通过率在数值上均未呈现下降的趋势。提出了一种测试医用电子直线加速器电子射野影像系统(EPID)探测板背散射屏蔽层效果的方法。Octavius 729和aSi-1200 EPID所使用软件计算的γ通过率无法可靠地识别引入误差的MLC射束模型,这些结果为解释IROC-H(Imaging and radiation oncology core houston quality assurance center)体模试验的高失败率提供了参考。

By comparing the application of aSi-1200 EPID and Octavius 729 detector using dose verification of conventional segmentation flattening filter free partial RapidArc plans,the limitations andγresponse to the modified multi-leaf collimator(MLC)model were investigated.CU(Calibration unit)value curves of Crossline and Inline in fluence plots for different square field plans designed on water phantom were used to examine the effectiveness of the backscattering shielding layer of an aSi-1200 EPID.A two-dimensional(2 D)γanalysis was performed on 50 plans under different tolerance criteria and thresholds(global normalization).The plan data with lowerγpass rate were imported into Verisoft software for dose reconstruction.By introducing transmission factor(TF)and dosimetric leaf gap(DLG)deviations with different magnitudes in multi-leaf collimator(MLC)model,a total of 80 modified dose distributions were obtained,and theγpassing rates between measured doses of the original plans and the TPS doses of the modified plans were analyzed.The results show that the CU values in the areas with dropped fields are 2~3 times higher in the positive off-axis positions,and the measured values in the Inline directions were generally smaller than the Crossline directions for a Varian EDGE linac.Nevertheless,CU values of the Halcyon accelerator coincided almost completely in both directions.The median and meanγpassing rates obtained from the EPID were higher than the 729 matrix.The deviations D D%between the TPS doses and the reconstructed doses based on measurements from the Octavius 729 and Octavius 1500 matrices are as follows:D 98%of PGTV(−9.60%,−14.30%),V_(30) of heart(−6.80%,−2.36%),V_(30) of Lung L(−8.11%,15.79%),V_(30) of Lung R(100%,50%),D_(max) of Cord(3.55%,−0.82%).With the increase in the deviations of TF and DLG,theγpass rates obtained from the EPID and Octavius 729 showed no trend of decreasing.A method to test the effectiveness of the backscattering shielding of an EPID detector performed using linac is provided.The dose measurement results suggested that theγindex obtained from the aSi-1200 EPID and Octavius 729 detectors could not reliably identify an error-introducing MLC model,which provide reference for the interpretation of the reported high failure rates of the IROC-H phantom tests.