The CyberKnife® (CK) Robotic Radiosurgery System as shown in Figure 11.1 is a stereotactic radiosurgery (SRS) device manufactured by Accuray, Inc. (Sunnyvale, CA, and USA) [1–3] that combines advanced robotic, image guidance, and linear accelerator (Linac) technologies to deliver high doses of ionizing radiation to lesions anywhere in the body [4–8]. It has a 6-MV Linac mounted on a robotic arm capable of pointing the radiation beam at the target from an increased number of noncoplanar directions. The robotic arm is spatially calibrated to an image-guidance system consisting of 2-kV X-ray generators and two digital detectors fixed on the ceiling and the floor of the treatment room, respectively. During treatment, the image guidance system takes images of the target region and compares them with corresponding digital reconstructed radiographs (DRRs) [9] obtained from the planning computed tomography (CT) scan. The calculated deviations of the position and orientation of the target are corrected automatically by adjusting the position and direction of each treatment beam [3].