Chojnowski Jacek M, Barnes Michael P, Sykes Jonathan R, Thwaites David I
Department of Radiation Oncology, Coffs Harbour Base Hospital, Coffs Harbour, NSW, Australia.
Institute of Medical Physics, School of Physics, University of Sydney, Sydney, NSW, Australia.
J Appl Clin Med Phys. 2017 Sep;18(5):178-183. doi: 10.1002/acm2.12147. Epub 2017 Aug 8.
Modern day Stereotactic treatments require high geometric accuracy of the delivered treatment. To achieve the required accuracy the IGRT imaging isocenter needs to closely coincide with the treatment beam isocenter. An influence on this isocenter coincidence and on the spatial positioning of the beam itself is the alignment of the treatment beam focal spot with collimator rotation axis. The positioning of the focal spot is dependent on the linac beam steering and on the stability of the monitor chamber and beam steering servo system. As such, there is the potential for focal spot misalignment and this should be checked on a regular basis. Traditional methods for measuring focal spot position are either indirect, inaccurate, or time consuming and hence impractical for routine use. In this study a novel, phantomless method has been developed using the EPID (Electronic Portal Imaging Device) that utilizes the different heights of the MLC and jaws. The method has been performed on four linear accelerators and benchmarked against an alternate ion chamber-based method. The method has been found to be reproducible to within ±0.012 mm (1 SD) and in agreement with the ion chamber-based method to within 0.001 ± 0.015 mm (1 SD). The method could easily be incorporated into a departmental routine linac QA (Quality Assurance) program.
现代立体定向治疗要求所给予的治疗具有很高的几何精度。为达到所需精度，图像引导放射治疗（IGRT）成像等中心需要与治疗束等中心紧密重合。治疗束焦点与准直器旋转轴的对准会影响这种等中心重合以及束本身的空间定位。焦点的定位取决于直线加速器束流控制以及监测电离室和束流控制伺服系统的稳定性。因此，存在焦点未对准的可能性，应该定期对此进行检查。传统的测量焦点位置的方法要么是间接的、不准确的，要么耗时，因此不适合常规使用。在本研究中，利用电子射野影像装置（EPID）开发了一种新的、无需体模的方法，该方法利用了多叶准直器（MLC）和限束器的不同高度。该方法已在四台直线加速器上进行，并与另一种基于电离室的方法进行了对比。已发现该方法的可重复性在±0.012毫米（1标准差）以内，并且与基于电离室的方法的一致性在0.001±0.015毫米（1标准差）以内。该方法可以很容易地纳入科室常规直线加速器质量保证（QA）程序中。
