Institute of Medical Physics, School of Physics, University of Sydney, Sydney, NSW 2006, Australia.
Med Phys. 2013 Sep;40(9):091902. doi: 10.1118/1.4816657.
Standard amorphous silicon electronic portal imaging devices (a-Si EPIDs) are x-ray imagers used frequently in radiotherapy that indirectly detect incident x-rays using a metal plate and phosphor screen. These detectors may also be used as two-dimensional dosimeters; however, they have a well-characterized nonwater-equivalent dosimetric response. Plastic scintillating (PS) fibers, on the other hand, have been shown to respond in a water-equivalent manner to x-rays in the energy range typically encountered during radiotherapy. In this study, the authors report on the first experimental measurements taken with a novel prototype PS a-Si EPID developed for the purpose of performing simultaneous imaging and dosimetry in radiotherapy. This prototype employs an array of PS fibers in place of the standard metal plate and phosphor screen. The imaging performance and dosimetric response of the prototype EPID were evaluated experimentally and compared to that of the standard EPID.
Clinical 6 MV photon beams were used to first measure the detector sensitivity, linearity of dose response, and pixel noise characteristics of the prototype and standard EPIDs. Second, the dosimetric response of each EPID was evaluated relative to a reference water-equivalent dosimeter by measuring the off-axis and field size response in a nontransit configuration, along with the off-axis, field size, and transmission response in a transit configuration using solid water blocks. Finally, the imaging performance of the prototype and standard EPIDs was evaluated quantitatively by using an image quality phantom to measure the contrast to noise ratio (CNR) and spatial resolution of images acquired with each detector, and qualitatively by using an anthropomorphic phantom to acquire images representative of human anatomy.
The prototype EPID's sensitivity was 0.37 times that of the standard EPID. Both EPIDs exhibited responses that were linear with delivered dose over a range of 1-100 monitor units. Over this range, the prototype and standard EPID central axis responses agreed to within 1.6%. Images taken with the prototype EPID were noisier than those taken with the standard EPID, with fractional uncertainties of 0.2% and 0.05% within the central 1 cm(2), respectively. For all dosimetry measurements, the prototype EPID exhibited a near water-equivalent response whereas the standard EPID did not. The CNR and spatial resolution of images taken with the standard EPID were greater than those taken with the prototype EPID.
A prototype EPID employing an array of PS fibers has been developed and the first experimental measurements are reported. The prototype EPID demonstrated a much morewater-equivalent dose response than the standard EPID. While the imaging performance of the standard EPID was superior to that of the prototype, the prototype EPID has many design characteristics that may be optimized to improve imaging performance. This investigation demonstrates the feasibility of a new detector design for simultaneous imaging and dosimetry treatment verification in radiotherapy.
标准非晶硅电子射野影像装置（a-Si EPID）是放射治疗中常用的 X 射线成像设备，它通过金属板和磷光屏间接探测入射 X 射线。这些探测器也可以用作二维剂量计；然而，它们具有特征明确的非水等效剂量响应。另一方面，塑料闪烁（PS）纤维已被证明在放射治疗中遇到的典型能量范围内以水等效的方式响应 X 射线。在这项研究中，作者报告了一种新型原型 PS a-Si EPID 的首次实验测量结果，该 EPID 是为在放射治疗中进行同时成像和剂量测定而开发的。该原型采用 PS 纤维阵列代替标准金属板和磷光屏。实验评估了原型 EPID 的成像性能和剂量响应，并将其与标准 EPID 进行了比较。
使用临床 6 MV 光子束首先测量原型和标准 EPID 的探测器灵敏度、剂量响应线性度和像素噪声特性。其次，通过在非传输配置下测量离轴和射野大小响应，以及在传输配置下使用实心水块测量离轴、射野大小和传输响应，评估每个 EPID 的剂量响应与参考水等效剂量计的关系。最后，使用图像质量体模定量评估原型和标准 EPID 的成像性能，以测量每个探测器获取的图像的对比度噪声比（CNR）和空间分辨率，并使用人体模型体模定性地获取代表人体解剖结构的图像。
原型 EPID 的灵敏度是标准 EPID 的 0.37 倍。两个 EPID 的响应在 1-100 个监测单位的范围内均与所传递的剂量呈线性关系。在这个范围内，原型和标准 EPID 的中心轴响应相差不到 1.6%。与标准 EPID 相比，原型 EPID 拍摄的图像噪声更大，中心 1cm²内的分数不确定度分别为 0.2%和 0.05%。对于所有剂量测量，原型 EPID 表现出近水等效的响应，而标准 EPID 则没有。标准 EPID 拍摄的图像的 CNR 和空间分辨率均大于原型 EPID 拍摄的图像。
已经开发了一种使用 PS 纤维阵列的原型 EPID，并报告了首次实验测量结果。原型 EPID 表现出比标准 EPID 更水等效的剂量响应。虽然标准 EPID 的成像性能优于原型，但原型 EPID 具有许多可优化以提高成像性能的设计特性。这项研究证明了在放射治疗中进行同时成像和剂量测定治疗验证的新型探测器设计的可行性。
