In radiotherapy, it is essential to give sucient dose to cancer without harming the normal tissue. Computed tomography (CT) has made it possible to see the position of the tumor deep in the body and has addressed the image-based radiotherapy in the 1980s. ree-dimensional (3D) radiotherapy planning (RTP) systems were rst quickly developed and then greatly expanded to use CT anatomical information to concentrate radiation dose to the tumor, using approaches such as 3D conformal radiotherapy (3D CRT), stereotactic body radiotherapy (SBRT), and intensity-modulated radiotherapy (IMRT). In the early 1990s, image guidance in the setup of patients on the treatment couch was realized to be an important step to register the virtual world in the treatment planning computer to the real world. However, tumors move in the body with physiological functions such as respiration, cardiac beat, digestion, and urination. e reliability of the tumor position as observed on static CT images and magnetic resonance imaging (MRI) has been questioned in the light of tumor motion studies performed in the late 1990s. For instance, in two studies, lung cancers and liver cancers near the diaphragm were shown to move not only in the craniocaudal direction but also in the anterodorsal and right-le lateral directions with amplitudes of 10-30 mm (Shimizu et al. 1999; 2000a, 2000b). Precise investigations in the early 2000s showed that the magnitude and trajectory of the motion diers among

patients, changes day by day in the same patient, and shis its baseline position even in a relatively short time such as one minute (Seppenwoolde et al. 2002; Shirato et al. 2004a, 2004b, 2006). ese results apparently imply that sharp dose gradients designed during the RTP phase for 3D CRT, SBRT, and IMRT approaches may be dierent from the real situation for tumors in moving organs.