## 3 Conceptual Design of Laser In-Frame Robotic Gantry  
The program of developing a new laser in-frame robot for very high-energy electron/photon beam therapy is to provide revolutionary equipment that overcomes some of the problems existing in the radiation therapy system, such as Cyber Knife [5], for instance, which is a combination of a small-sized X-ray linac and a traditional industrial manipulator in its structure. Intense laser pulses are generated and transmitted to the base of the manipulator, then through its frame and joints, and are finally delivered to the accelerator equipped at the tip of the manipulator, where the electron beam is generated. A laser beam with a diameter of the order of 50 mm has to be transmitted in vacuum before it is delivered to the accelerator. Therefore the design feature of the robot arm should meet all the requirements on payload, positioning accuracy, and an inside vacuum hollow structure with minimum 80 mm-level diameter, as well as keeping the manufacturing costs as low as possible. In our proposal, all the frames and joints are designed in a large-sized internal vacuum hollow structure and then all of the driven components have to be outward distributed. It is clear that five is the minimum necessary number of DOFs (degrees of freedom) for the treatment by electron/photon beam to cancer cells, by adjusting the position and pose of the beam. The robot may be capable of providing five or fewer DOFs, and other equipment, such as the treatment couch, may contribute the rest (one or more DOFs). For medical use, the robotic arm is also required to be small, compact, hygienic, and noiseless. The five DOFs are waist rotation, elbow pitch, and shoulder pitch, as well as the tilts of the wrist with two further revolute joints. Among them, the rotary joint (especially for waist rotation), which acts as an essential driving/guiding component and supporting structure, is of extreme importance. At the beginning of the treatment, the joints will find their relative location of the lesion programmed by the computer. During the treatment, the irradiated point should be adjusted immediately once the data of the location of the lesion changes. This system must be real-time and accurate-positioning. This rotary joint needs to have a vacuum channel inside, the diameter of which should be at least 80 mm to serve for the laser transmission. There is no such commercial mechanism that meets these requirements among the present industrial manipulators. Therefore, the design is an extremely difficult and challenging task compared to a traditional manipulator. Here we propose a low cost but high positioning accuracy rotary joint, thereby reducing the cost of the package of this medical robotic system. The rotary joint is able to withstand the weight of the whole robotic arm device itself as well as the external load. Fast and accurate positioning to the pathological location determined by the medical plan is promised. In the course of treatment, the end of the robot arm is therefore able to keep changing subtly at any time according to the positioning data, and to adjust the irradiation site to achieve accuracy of real-time treatment. The main task of our research is developing a new laser in-frame robot with five degrees of freedom to guide the laser beam inside. For medical use, the robotic arm is required to be small and compact to ensure the laser beam positioning accuracy at the manipulator end as well as the lowest possible cost. In addition, the robotic arm shall maintain an internal hollow for implantation of a vacuum optical path. The design of the robotic rotary joint is one of the most crucial steps to realize these objectives. Therefore, the design features of the rotary joint should be able to meet the requirements of the work load of the robot arm and the accelerator, the rotational positioning accuracy of the rotary mechanism, as well as reducing the manufacturing cost of the entire robotic system. Compared to traditional manipulator design, this is a difficult and challenging task.