Barkhof J, Schut G, Flanz J B, Goitein M, Schippers J M
Kernfysisch Versneller Instituut, Groningen, The Netherlands.
Med Phys. 1999 Nov;26(11):2429-37. doi: 10.1118/1.598761.
An easily-used system has been developed for routine measurements of the alignment of beams used for radiation therapy. The position of a beam of circular cross section is measured with respect to a steel sphere fixed to the patient positioning table and which should coincide with the isocenter. Since measurements can be done at all gantry angles (if one is available) and with all possible orientations of the patient table, the system is particularly suited for rapid and accurate measurements of gantry and/or couch isocentricity. Because it directly measures beam-to-positioner offset, the system provides an inclusive alignment verification of the total treatment system. The system has been developed for use with proton beams, but it could equally be used for alignment checks of an x-ray beam from a linear accelerator or other source. The measuring instrument consists of a scintillation screen viewed by a CCD camera, mounted on the gantry downstream of the sphere. The steel sphere is not large enough to stop protons of all energies of interest; however, it will always modify the energy and direction of protons which intersect it, creating a region of lower intensity (a "shadow") in the light spot created by the proton beam hitting the screen. The position of the shadow with respect to the light spot is a measure of the alignment of the system. An image-analysis algorithm has been developed for an automatic determination of the position of the shadow with respect to the light spot. The specifications and theoretical analysis of the system have been derived from Monte Carlo simulations, which are validated by measurements. We have demonstrated that the device detects beam misalignments with an accuracy (1 s.d.) of 0.05 mm, which is in agreement with the expected performance. This accuracy is more than sufficient to detect the maximum allowed misalignment of +/-0.5 mm.
已开发出一种易于使用的系统，用于常规测量放射治疗所用束流的对准情况。对于固定在患者定位台上且应与等中心重合的钢球，测量圆形横截面束流的位置。由于可以在所有机架角度（如果有可用角度）以及患者床的所有可能方位进行测量，该系统特别适合快速且准确地测量机架和/或治疗床的等中心性。因为它直接测量束流与定位器的偏移，所以该系统提供了整个治疗系统的全面对准验证。该系统是为与质子束配合使用而开发的，但同样可用于对来自直线加速器或其他源的X射线束进行对准检查。测量仪器由一个闪烁屏和一台CCD相机组成，相机安装在球状物下游的机架上。钢球不够大，无法阻挡所有感兴趣能量的质子；然而，它总会改变与它相交的质子的能量和方向，在质子束撞击屏幕产生的光斑中形成一个强度较低的区域（一个“阴影”）。阴影相对于光斑的位置是系统对准情况的一种度量。已开发出一种图像分析算法，用于自动确定阴影相对于光斑的位置。该系统的规格和理论分析源自蒙特卡罗模拟，并通过测量进行了验证。我们已证明，该设备检测束流未对准的精度（1个标准差）为0.05毫米，这与预期性能相符。这种精度足以检测出最大允许的±0.5毫米未对准情况。
