ABSTRACT
Since the discovery of x-rays by Rontgen in 1895, x-ray imaging has been widely used in medical applications. The advent of x-ray computed tomography (CT) imaging was one of the most significant advancements in radiography. CT images are obtained by rotating a highly collimated x-ray source around the patient and having detectors record projection x-ray data from a multitude of angles throughout the rotation. Tomographic image reconstruction techniques are then used to create an axial cross-section image of the irradiated volume of the patient. Three-dimensional (3D) images can be constructed by translating the patient through the plane of the x-ray beam in either a stepwise or a continuous motion, resulting in axial or helical scans, respectively, which can be combined in a single 3D image set. Volumetric patient CT images are commonly used in radiotherapy treatment planning. In recent years, a new technique in imaging-guided radiotherapy (IGRT) is kV cone-beam computerized tomography (CBCT) (Uematsu et al., 1996; Jaffray et al., 1999, 2002; Jaffray and Siewerdsen, 2000; Siewerdsen and Jaffray, 2000; Shiu et al., 2003; Yenice et al., 2003; Sharpe et al., 2006) of the patient on the treatment table. CBCT is capable of providing accurate 3D volumetric knowledge about the patient’s anatomy for every treatment fraction and is also suitable for adaptive corrections of errors related to interfractional uncertainties of the treatment process. With the availability of in-room kV-CBCT, it becomes practical to monitor the patient treatment positions and tumor shrinkage on a daily or weekly basis as well as to perform potentially adaptive radiotherapy (ART) (Ding et al., 2007b; Pawlowski et al., 2010) and IGRT (Jaffray et al., 1999; Jaffray and Siewerdsen, 2000).
