Department of Radiation Oncology, UT Southwestern Medical Center, Dallas, Texas 75390, USA.
Med Phys. 2011 May;38(5):2335-41. doi: 10.1118/1.3570768.
Several linacs with integrated kilovoltage (kV) imaging have been developed for delivery of image guided radiation therapy (IGRT). High geometric accuracy and coincidence of kV imaging systems and megavoltage (MV) beam delivery are essential for successful image guidance. A geometric QA tool has been adapted for routine QA for evaluating and characterizing the geometric accuracy of kV and MV cone-beam imaging systems. The purpose of this work is to demonstrate the application of methodology to routine QA across three IGRT-dedicated linac platforms.
It has been applied to a Varian Trilogy (Varian Medical Systems, Palo Alto, CA), an Elekta SynergyS (Elekta, Stockholm, Sweden), and a Brainlab Vero (Brainlab AG, Feldkirchen, Germany). Both the Trilogy and SynergyS linacs are equipped with a retractable kV x-ray tube and a flat panel detector. The Vero utilizes a rotating, rigid ring structure integrating a MV x-ray head mounted on orthogonal gimbals, an electronic portal imaging device (EPID), two kV x-ray tubes, and two fixed flat panel detectors. This dual kV imaging system provides orthogonal radiographs, CBCT images, and real-time fluoroscopic monitoring. Two QA phantoms were built to suit different field sizes. Projection images of a QA phantom were acquired using MV and kV imaging systems at a series of gantry angles. Software developed for this study was used to analyze the projection images and calculate nine geometric parameters for each projection. The Trilogy was characterized five times over one year, while the SynergyS was characterized four times and the Vero once. Over 6500 individual projections were acquired and analyzed. Quantitative geometric parameters of both MV and kV imaging systems, as well as the isocenter consistency of the imaging systems, were successfully evaluated.
A geometric tool has been successfully implemented for calibration and QA of integrated kV and MV across a variety of radiotherapy platforms. X-ray source angle deviations up to 0.8 degrees, and detector center offsets up to 3 mm, were observed for three linacs, with the exception of the Vero, for which a significant center offset of one kV detector (prior to machine commissioning) was observed. In contrast, the gimbal-based MV source positioning of the Vero demonstrated differences between observed and expected source positions of less than 0.2 mm, both with and without gimbal rotation.
This initial application of this geometric QA tool shows promise as a universal, independent tool for quantitative evaluation of geometric accuracies of both MV and integrated kV imaging systems across a range of platforms. It provides nine geometric parameters of any imaging system at every gantry angle as well as the isocenter coincidence of the MV and kV image systems.
已经开发了几种带有集成千伏 (kV) 成像的直线加速器，用于提供图像引导放射治疗 (IGRT)。kV 成像系统和兆伏 (MV) 射束传输的高几何精度和一致性对于成功的图像引导至关重要。已经为常规 QA 适应了一种几何 QA 工具，用于评估和描述 kV 和 MV 锥形束成像系统的几何精度。本工作的目的是展示该方法在三个 IGRT 专用直线加速器平台上的常规 QA 中的应用。
它已应用于瓦里安 Trilogy（瓦里安医疗系统，帕洛阿尔托，加利福尼亚州）、医科达 SynergyS（医科达，斯德哥尔摩，瑞典）和 Brainlab Vero（Brainlab AG，菲尔德基希海姆，德国）。Trilogy 和 SynergyS 直线加速器都配备了可伸缩的 kV X 射线管和平板探测器。Vero 则采用旋转刚性环结构，集成了安装在正交万向架上的 MV X 射线头、电子射野成像装置 (EPID)、两个 kV X 射线管和两个固定的平板探测器。这个双 kV 成像系统提供了正交射线照相、CBCT 图像和实时荧光透视监测。为了适应不同的射野大小，构建了两个 QA 体模。使用 MV 和 kV 成像系统在一系列旋转机架角度下获取 QA 体模的投影图像。用于这项研究的软件用于分析投影图像并为每个投影计算九个几何参数。 Trilogy 在一年中进行了五次特征描述，而 SynergyS 进行了四次，Vero 进行了一次。共采集和分析了超过 6500 个单独的投影。成功评估了集成 kV 和 MV 成像系统的定量几何参数以及成像系统的等中心一致性。
已经成功实施了一种几何工具，用于校准和常规 QA，以跨多种放射治疗平台进行集成 kV 和 MV。除了 Vero 之外，对于三个直线加速器，观察到的 X 射线源角度偏差高达 0.8 度，探测器中心偏移高达 3 毫米，而 Vero 则观察到一个重要的 kV 探测器的中心偏移（在机器调试之前）。相比之下，Vero 的基于万向架的 MV 源定位显示，无论是否旋转万向架，观察到的和预期的源位置之间的差异都小于 0.2 毫米。
这种几何 QA 工具的初步应用显示出作为一种通用的、独立的工具的潜力，用于定量评估各种平台上的 MV 和集成 kV 成像系统的几何精度。它为每个旋转机架角度的任何成像系统提供了九个几何参数以及 MV 和 kV 图像系统的等中心一致性。
