## 1 Introduction  
Radiation therapy uses high energy radiation such as X-rays, gamma-rays, electrons, protons, heavy ions, and neutrons to kill cancer cells by damaging their DNA. Since radiation therapy can damage normal cells as well as cancer cells, treatment must be carefully planned to minimize such side effects. External-beam radiation therapy is most often delivered in the form of photon beams--either X-rays or gamma rays from a machine outside the body--in contrast to internal radiation therapy or brachytherapy, which use radioactive material placed in the body near cancer cells [1]. Since electron beams have a finite range after which the dose falls off rapidly in order to spare deeper normal tissue, electron beam therapy is typically used in the treatment of superficial tumors like cancer of skin regions or total skin, diseases of the limbs, and so on. For deeper regions, intra-operative electron radiation therapy might be applied. Many types of external-beam radiation therapy are delivered using a machine called a medical linear accelerator driven by RF power amplifiers such as magnetron and klystron, producing electron beams with an energy range of 6-20 MeV [2]. Technical advances in external-beam radiation therapy provide effective shaping of three-dimensional dose distributions and spatially accurate dose delivery by imaging verification. These technologies, including three-dimensional conformal radiation therapy (3D-CRT), intensity-modulated radiation therapy (IMRT), image guided radiation therapy (IGRT), stereotactic body radiation therapy (SBRT), and hadron therapy, increase the radiation dose to tumor areas and reduce radiation exposure to specific sensitive areas of surrounding normal tissue. In particular, the SBRT provides the radiation in a shorter time using smaller radiation fields and higher doses compared with the 3D-CRT by means of very sophisticated computer software and advanced treatment machines to deliver radiation very precisely to target areas in combination with a robotic manipulator and compact medical accelerator [1]. Medical robots are becoming revolutionary tools for a variety of clinical applications because of capabilities of exceeding human limitations on tremor reduction, repeatability, precision, and accuracy. In several areas of medicine, robots have been successfully developed and exploited: in 1985, a robot, the PUMA 560, was used to place a needle for a brain biopsy using CT guidance; in 1988, the PROBOT developed at Imperial College London was used to perform prostatic surgery by [PERSON] at Guy's and St. Thomas' Hospital, London [3]; tele-operated robots called da Vinci (Intuitive Surgical Inc., Sunnyvale, CA, USA) have been applied to cardiac and abdominal surgery, allowing soft tissue procedures to benefit from robotic enhancement [4]; and the Cyber Knife radiosurgery system (Accuray TM, Inc, Sunnyvale, CA, USA) was first used for treating various tumors by delivering non-isocentric radiation beam from a compact linac mounted on a robot [5; 6; 7]. The Cyber Knife was said to be the first robotic device in which a human was permitted to be present within a robot workspace. Several generations of the Cyber Knife system have been successfully developed and applied since its initial inception in 1990. However, the limited penetration depth and low quality of the transverse penumbra at the energy range of 6-20 MeV delivered from present medical RF linear accelerators currently prevent the aforementioned advanced modalities from being used in cancer treatment. These drawbacks can be overcome if the electron energy is increased above 50 MeV, whereby the penetration depth becomes longer and the transverse penumbra sharper, though the longitudinal penumbra is also increased.