Rowshanfarzad P, Riis H L, Zimmermann S J, Ebert M A
1 School of Physics, University of Western Australia, Crawley, WA, Australia.
2 Radiofysisk Laboratorium, Odense University Hospital, Odense, Denmark.
Br J Radiol. 2015 Jul;88(1051):20140581. doi: 10.1259/bjr.20140581. Epub 2015 Apr 23.
In radiotherapy treatments, it is crucial to monitor the performance of linear accelerator (linac) components, including gantry, collimation system and electronic portal imaging device (EPID) during arc deliveries. In this study, a simple EPID-based measurement method is suggested in conjunction with an algorithm to investigate the stability of these systems at various gantry angles with the aim of evaluating machine-related errors in treatments.
The EPID sag, gantry sag, changes in source-to-detector distance (SDD), EPID and collimator skewness, EPID tilt and the sag in leaf bank assembly owing to linac rotation were separately investigated by acquisition of 37 EPID images of a simple phantom with 5 ball bearings at various gantry angles. A fast and robust software package was developed for automated analysis of the image data. Nine Elekta AB (Stockholm, Sweden) linacs of different models and number of years in service were investigated.
The average EPID sag was within 2 mm for all tested linacs. Some machines showed >1-mm gantry sag. Changes in the SDD values were within 1.3 cm. EPID skewness and tilt values were <1° in all machines. The maximum sag in multileaf collimator leaf bank assemblies was around 1 mm. A meaningful correlation was found between the age of the linacs and their mechanical performance. Conclusions and Advances in knowledge: The method and software developed in this study provide a simple tool for effective investigation of the behaviour of Elekta linac components with gantry rotation. Such a comprehensive study has been performed for the first time on Elekta machines.
在放射治疗中，在弧形照射期间监测直线加速器（直线加速器）组件的性能至关重要，这些组件包括机架、准直系统和电子射野影像装置（EPID）。在本研究中，提出了一种基于EPID的简单测量方法，并结合一种算法来研究这些系统在不同机架角度下的稳定性，旨在评估治疗中与机器相关的误差。
通过在不同机架角度采集一个带有5个滚珠轴承的简单模体的37张EPID图像，分别研究了EPID下垂、机架下垂、源到探测器距离（SDD）的变化、EPID和准直器偏斜、EPID倾斜以及由于直线加速器旋转导致的叶片库组件下垂。开发了一个快速且强大的软件包用于图像数据的自动分析。研究了9台不同型号和使用年限的医科达公司（瑞典斯德哥尔摩）直线加速器。
所有测试直线加速器的EPID平均下垂在2毫米以内。一些机器的机架下垂超过1毫米。SDD值的变化在1.3厘米以内。所有机器的EPID偏斜和倾斜值均小于1°。多叶准直器叶片库组件的最大下垂约为1毫米。发现直线加速器的使用年限与其机械性能之间存在有意义的相关性。知识结论与进展：本研究中开发的方法和软件为有效研究医科达直线加速器组件随机架旋转的行为提供了一个简单工具。首次对医科达机器进行了如此全面的研究。
