Chytyk K, McCurdy B
Medical Physics Division, CancerCare Manitoba, Winnipeg, MB.
Department of Physics and Astronomy, University of Manitoba, Winnipeg, MB.
Med Phys. 2008 Jul;35(7Part2):3399. doi: 10.1118/1.2965911.
Amorphous silicon (a-Si) electronic portal imaging devices (EPIDs) have been heavily investigated as treatment verification tools, with a particular focus on intensity modulated radiation therapy (IMRT). This verification could be accomplished through a comparison of measured portal images to predicted images. A general fluence determination for portal dose image prediction would be a great asset in order to model the complex modulation of IMRT. A physically-based parameter fluence model was developed by matching multi-leaf collimator defined predicted images to measured image profiles. The fluence model was composed of a focal Gaussian and extrafocal Gaussian-like source (Pearson VII). Specific aspects of the MLCs and secondary collimators were also modeled (eg. jaw and MLC transmission factors, MLC rounded leaf tips, tongue and groove effect, interleaf leakage, MLC offsets). The resulting calculated fluence was then convolved with Monte Carlo generated EPID-specific dose kernels to convert incident fluence to dose delivered to the EPID. Measured EPID data was obtained with an a-Si EPID for various MLC-defined fields (1×1 to 20×20 cm ) over a range of source-to-imager distances. These measured profiles were used to determine the fluence model parameters and the resulting model was tested on prostate and oropharyngeal IMRT fields. The model predicted the open-field profiles within 2%, 2mm, while the predicted IMRT fields were generally within 3%, 3mm for at least 96% of the pixels. This model demonstrates the necessary accuracy needed for IMRT portal dose image prediction in complex clinical examples (<3%, 3mm) and could be used for pre-treatment verification.
非晶硅（a-Si）电子射野影像装置（EPID）作为治疗验证工具已得到深入研究，尤其关注调强放射治疗（IMRT）。这种验证可通过将实测射野影像与预测影像进行比较来完成。为模拟IMRT的复杂调制，确定用于射野剂量影像预测的通用注量将是一项重要资产。通过将多叶准直器定义的预测影像与实测影像轮廓相匹配，开发了一种基于物理的参数注量模型。该注量模型由一个焦点高斯源和焦点外类高斯源（皮尔逊VII型）组成。还对多叶准直器和二级准直器的具体方面进行了建模（例如，准直器钳口和多叶准直器的透射因子、多叶准直器圆形叶尖、舌槽效应、叶片间泄漏、多叶准直器偏移）。然后将所得计算出的注量与蒙特卡罗生成的EPID特定剂量核进行卷积，以将入射注量转换为传递到EPID的剂量。使用a-Si EPID在一系列源到成像器距离上针对各种多叶准直器定义的射野（1×1至20×20 cm）获取实测EPID数据。这些实测轮廓用于确定注量模型参数，并在前列腺和口咽IMRT射野上对所得模型进行测试。该模型预测开放射野轮廓在2%、2mm范围内，而预测的IMRT射野对于至少96%的像素通常在3%、3mm范围内。该模型展示了复杂临床实例中IMRT射野剂量影像预测所需的必要精度（<3%，3mm），可用于治疗前验证。
