VARIAN Clinac iX直線加速器之OBI系統品質保證

Abstract

The imaging-guided radiation therapy (IGRT) in radiotherapy is getting to popular today. It used the imaging-guided system to verify the patient setup position before the daily treatment. The guided images also can fuse with the digitally reconstructed radiograph (DRR) of the CT scan to enhance and the accuracy of each treatment. The purpose of the study is to evaluate the quality assurance (QA) of the Varian Clinac iX’s On-Board Imager® (OBI) system by using the criteria of Yoo’s paper (2006), which classified the QA into three parts: 1.safety and functionality, 2.geometry and 3.image quality. The first part is to assure the hardware with software of the OBI system and the warning device worked fine. The second one is to confirm the OBI’s isocenter consistent with the treatment center, and to check the accuracy of the auto alignment correction and the couch movement. There are two image modalities of OBI system: 2D radiography and 3D image by cone beam computed tomography (CBCT), where the image QA was also performed. The 2D radiographic image QA includes the items of spatial resolution and contrast. The CBCT has two modes need to be checked: one is the Full-fan mode with head scan parameters, and the other is the Half-fan mode with pelvis scan parameters. The items of 3D image QA including Hounsfield Units’ (HU) linearity and uniformity, low contrast, spatial resolution, spatial linearity and slice thickness. The QA of safety and functionality are within our criteria. About the QA of geometry, the average deviation of OBI’s isocenter is +0.13 mm in X-axis, -0.44 mm in Y-axis and +0.63 mm in Z-axis. About the 2D image QA, the contrast range was between 2.2% and 1.7%, and the spatial resolution is 1.6 lp/cm. About the 3D image QA, the low contrast, spatial resolution, and spatial linearity are all within the criteria for both of the two modes. IGRT can enhance the geometric accuracy of treatment; therefore the tumor control probability can be improved and the normal tissues complication probability can also be reduced. However, in order to give patient an accurate treatment, the QA of image-guided system is surely very important, which we discussed comprehensively in this paper.