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PURPOSE

This study presents novel quality assurance (QA) approach for volumetric modulated arc therapy (VMAT) that leverages frame-by-frame electronic portal imaging device (EPID) images integrated into Mobius3D for accurate three-dimensional dose calculations.

METHODS

Sequential EPID images for VMAT plans were acquired every 0.4-second by iView system and processed through iterative deconvolution to mitigate blurring from photon scattering. Deconvolved images were binarized to define multi-leaf collimator (MLC) positions. Pre-acquired box fluences determined optimal threshold for binarization and adjusted for detector shift depending on gantry and collimator angles. Sequential EPID images were re-scaled using pixel scaling factor (PSF) and converted to monitor unit (MU) proportional values. Generated EPID-based log file, including control-point specific MLC and monitor units (MU) information, were analyzed in Mobius3D for Gamma passing rate (GPR) of VMAT plans from 18 patients. Plan complexity indices were calculated and correlated with GPR.

RESULTS

Clinically appropriate threshold was defined to be 20000 that can extract accurate MLC data from the deconvolved binarized EPID images. Positional deviations due to gantry and collimator rotations were observed to be up to 4.5 pixels. Recalibrated EPID pixel values showed linearity with MU regardless of changes in dose rate. Consequently, average GPR for 18 patients evaluated using Mobius3D reached 95.2% ± 3.7%%, based on 3% dose difference and 3mm distance-to-agreement criterion. It was found that two plan complexity indices showed statistically significant correlation with GPR.

CONCLUSION

This study successfully implemented novel measurement-based VMAT QA framework based on control-point specific EPID, based upon accurate MLC and MU data at each frame.

PURPOSE

METHODS

RESULTS

CONCLUSION

This study successfully implemented novel measurement-based VMAT QA framework based on control-point specific EPID, based upon accurate MLC and MU data at each frame.

目的

本研究提出了一种用于容积调强弧形治疗（VMAT）的新型质量保证（QA）方法，该方法利用逐帧电子射野影像装置（EPID）图像集成到Mobius3D中，以进行精确的三维剂量计算。

方法

通过iView系统每0.4秒采集VMAT计划的连续EPID图像，并通过迭代去卷积进行处理，以减轻光子散射造成的模糊。对去卷积后的图像进行二值化处理，以定义多叶准直器（MLC）的位置。预先获取的盒式注量确定了二值化的最佳阈值，并根据机架和准直器角度对探测器偏移进行调整。使用像素缩放因子（PSF）对连续的EPID图像进行重新缩放，并转换为监测单位（MU）比例值。在Mobius3D中分析生成的基于EPID的日志文件，包括控制点特定的MLC和监测单位（MU）信息，以计算18例患者VMAT计划的伽马通过率（GPR）。计算计划复杂性指数，并将其与GPR相关联。

结果

临床适用阈值定义为20000，该阈值可从去卷积后的二值化EPID图像中提取准确的MLC数据。观察到由于机架和准直器旋转导致的位置偏差高达4.5像素。重新校准的EPID像素值显示与MU呈线性关系，而与剂量率变化无关。因此，基于3%剂量差异和3mm距离一致性标准，使用Mobius3D评估的18例患者的平均GPR达到95.2%±3.7%。发现两个计划复杂性指数与GPR具有统计学显著相关性。