Du Weiliang, Gao Song, Jiang Wei, Kudchadker Rajat J
Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, TX, USA.
Yantai Yuhuangding Hospital, Qingdao University School of Medicine, Yantai, Shandong, China.
J Appl Clin Med Phys. 2018 Sep;19(5):483-490. doi: 10.1002/acm2.12402. Epub 2018 Jun 29.
Modern medical linear accelerators (linacs) are often equipped with image guidance systems that are capable of megavolt (MV), kilovolt (kV), planar, or volumetric imaging. On Varian TrueBeam linacs, the isocenter accuracies of the imaging systems are calibrated with a procedure named IsoCal. On Clinac series linacs from Varian, installation of IsoCal is optional and the effects of IsoCal on the imaging systems can be turned on or off after the IsoCal procedure is performed. In this study, we report on the effectiveness of IsoCal in improving the coincidence of the image centers with the radiation isocenter, using an independent Winston-Lutz (WL) method to locate the radiation isocenter. A ball-bearing phantom was imaged with 2D MV, 2D kV, and cone beam computed radiography systems on two TrueBeam and two Clinac machines. Using the same phantom, digital WL tests with 16 combinations of gantry and collimator angles were performed to locate the radiation isocenter. The offsets between the IsoCal-calibrated image centers and the WL radiation isocenter were found to be within 0.4 mm on the four linacs in this study. When IsoCal was turned off, the maximal offsets of the image centers were greater than 1.0 mm on the two Clinac machines. The method developed in this study can be used as a vendor-independent quality assurance tool to assess the isocentricity of the image centers and radiation central axes.
现代医用直线加速器（直线加速器）通常配备有能够进行兆伏（MV）、千伏（kV）、平面或容积成像的图像引导系统。在瓦里安TrueBeam直线加速器上，成像系统的等中心精度通过一种名为IsoCal的程序进行校准。在瓦里安Clinac系列直线加速器上，IsoCal的安装是可选的，并且在执行IsoCal程序后，可以打开或关闭IsoCal对成像系统的影响。在本研究中，我们报告了使用独立的温斯顿-卢茨（WL）方法定位辐射等中心时，IsoCal在提高图像中心与辐射等中心重合度方面的有效性。使用两个TrueBeam和两个Clinac机器上的二维MV、二维kV和锥束计算机射线摄影系统对一个滚珠轴承体模进行成像。使用相同的体模，进行了16种机架和准直器角度组合的数字WL测试，以定位辐射等中心。在本研究中的四台直线加速器上，IsoCal校准的图像中心与WL辐射等中心之间的偏移量被发现小于0.4毫米。当关闭IsoCal时，两台Clinac机器上图像中心的最大偏移量大于1.0毫米。本研究中开发的方法可作为一种独立于供应商的质量保证工具，用于评估图像中心和辐射中心轴的等中心性。
