The most important geometric characteristic of stereotactic treatment is the accuracy of positioning the target at the treatment isocenter and the accuracy of directing the radiation beam at the treatment isocenter. Commonly, the radiation isocenter is used as the reference for the treatment isocenter, but its method of localization is not strictly defined, and it depends on the linac-specific beam steering parameters. A novel method is presented for determining the linac mechanical isocenter position and size based on the localization of the collimator axis of rotation at arbitrary gantry angle. The collimator axis of rotation position is determined from the radiation beam center position corrected for the focal spot offset. The focal spot offset is determined using the image center shift method with a custom-design rigid phantom with two sets of ball-bearings. Three specific quality assurance (QA) applications and assessment methods are also presented to demonstrate the functionality of linac mechanical isocenter position and size determination in clinical practice. The first is a mechanical and radiation isocenters coincidence test suitable for quick congruence assessment of these two isocenters for a selected energy, usually required after a nonroutine linac repair and/or energy adjustment. The second is a stereotactic beam isocentricity assessment suitable for pretreatment stereotactic QA. The third is a comprehensive linac geometrical performance test suitable for routine linac QA. The uncertainties of the method for determining mechanical isocenter position and size were measured to be 0.05 mm and 0.04 mm, respectively, using four available photon energies, and were significantly smaller than those of determining the radiation isocenter position and size, which were 0.36 mm and 0.12 mm respectively. It is therefore recommended that the mechanical isocenter position and size be used as the reference linac treatment isocenter and a linac mechanical characteristic parameter respectively.

中文翻译：

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一种确定直线加速器机械等中心位置和尺寸的新方法以及特定 QA 应用的示例

立体定向治疗最重要的几何特征是在治疗等中心定位目标的准确性和在治疗等中心引导辐射束的准确性。通常，辐射等中心被用作治疗等中心的参考，但其定位方法并没有严格定义，它取决于直线加速器特定的光束控制参数。提出了一种基于准直仪旋转轴在任意机架角度的定位来确定直线加速器机械等中心位置和尺寸的新方法。准直器的旋转轴位置由针对焦斑偏移校正的辐射束中心位置确定。焦斑偏移是使用图像中心偏移方法通过定制设计的刚性体模和两组滚珠轴承确定的。还介绍了三种特定的质量保证 (QA) 应用程序和评估方法，以证明直线加速器机械等中心位置和尺寸确定在临床实践中的功能。第一个是机械和辐射等中心重合测试，适用于对选定能量的这两个等中心进行快速一致评估，通常在非常规直线加速器修复和/或能量调整后需要。第二个是适用于预处理立体定向 QA 的立体定向光束等心度评估。第三是适合常规直线加速器质量保证的综合直线加速器几何性能测试。使用四种可用光子能量测得的确定机械等中心位置和尺寸的方法的不确定度分别为 0.05 mm 和 0.04 mm，并且明显小于确定辐射等中心位置和尺寸的方法的不确定度，分别为 0.36 mm 和 0.04 mm。分别为 0.12 毫米。因此，建议将机械等中心位置和大小分别用作参考直线加速器治疗等中心点和直线加速器机械特性参数。