Zacharopoulos Nicholas G, Fenyes David A
Aktina Medical Corp, Congers, New York, USA.
Med Phys. 2023 May;50(5):2672-2682. doi: 10.1002/mp.16341. Epub 2023 Mar 21.
The traditional approach to medical linear accelerator (LINAC) isocenter quality assurance is to compare the radius of the LINAC isocenter, determined via analysis of Winston-Lutz (WL) dataset, to a threshold in order to approve the LINAC for clinical use. This scalar metric provides little insight into beam-to-target accuracy with gantry and couch motion.
To develop a method for verifying isocenter with increased sensitivity over traditional WL techniques by accounting for geometric errors that could normally be overlooked.
From WL images, we construct radiation beam axis and marker shift locations in a 3D coordinate system. These axes and shift positions are used to construct a new isocenter performance metric, the -matrix, which predicts direction and magnitude of beam-to-target errors across combinations of couch and gantry position. We introduce clinical isocenter as the location optimizing a cost function derived from the -matrix, which serves as an optimal target for tumor positioning. We demonstrated these techniques on a clinical LINAC with an initial randomly positioned marker which was subsequently repositioned based on the optimized clinical isocenter location. The marker shifts, radiation isocenter, and -matrix were compared before and after repositioning. We compared the new technique against typically used WL techniques using a monte carlo simulation modeling variations in LINAC geometry, marker position, and measurement noise.
The technique was successfully demonstrated on a Varian LINAC. Marker repositioning to clinical isocenter yielding an error matrix with magnitudes all below 0.81 mm. As expected, marker position had little impact on the radiation isocenter location and radius, and also had little impact on clinical isocenter location. The verification results show the accuracy of the -matrix to predict beam to tumor geometric inaccuracies. The Monte Carlo simulations demonstrate that the -matrix is more sensitive and specific for detecting potential treatment errors compared to the traditional WL techniques.
We have developed and demonstrated the usefulness of a framework for verifying isocenter based on a 3D model of the radiation beam axes and tumor movement from couch rotations, derived from 2D WL transmission images. The -matrix replaces the scalar isocenter radius as a metric of isocenter quality, providing insight into contribution of couch and radiation beams to isocenter quality, and exposing treatment errors ignored by the traditional method. The clinical isocenter provides an alternate to physical isocenter as a target for tumor positioning in cases of suboptimal LINAC geometry.
医学直线加速器（LINAC）等中心质量保证的传统方法是将通过分析温斯顿 - 卢茨（WL）数据集确定的LINAC等中心半径与阈值进行比较，以批准LINAC用于临床。这种标量指标几乎无法深入了解机架和治疗床运动时射束与靶点的准确性。
通过考虑通常可能被忽视的几何误差，开发一种比传统WL技术具有更高灵敏度的等中心验证方法。
从WL图像中，我们在三维坐标系中构建辐射束轴和标记物偏移位置。这些轴和偏移位置用于构建一个新的等中心性能指标，即误差矩阵，它可以预测治疗床和机架位置组合下射束与靶点误差的方向和大小。我们引入临床等中心作为优化从误差矩阵导出的成本函数的位置，该成本函数作为肿瘤定位的最佳靶点。我们在一台临床LINAC上展示了这些技术，初始时标记物随机定位，随后根据优化后的临床等中心位置重新定位。比较重新定位前后的标记物偏移、辐射等中心和误差矩阵。我们使用蒙特卡罗模拟对LINAC几何形状、标记物位置和测量噪声的变化进行建模，将新技术与常用的WL技术进行比较。
该技术在瓦里安LINAC上成功得到验证。将标记物重新定位到临床等中心后，得到的误差矩阵大小均低于0.81毫米。正如预期的那样，标记物位置对辐射等中心位置和半径影响很小，对临床等中心位置影响也很小。验证结果表明误差矩阵能够准确预测射束到肿瘤的几何误差。蒙特卡罗模拟表明，与传统WL技术相比，误差矩阵在检测潜在治疗误差方面更敏感、更具特异性。
我们已经开发并证明了一个基于辐射束轴和治疗床旋转引起的肿瘤运动的三维模型来验证等中心的框架的实用性，该模型源自二维WL透射图像。误差矩阵取代了标量等中心半径作为等中心质量的指标，深入了解治疗床和辐射束对等中心质量的贡献，并揭示传统方法忽略的治疗误差。在LINAC几何形状欠佳的情况下，临床等中心为肿瘤定位提供了一个替代物理等中心的靶点。
