Applications of X-band Technology in Medical Accelerators

Most radiation therapy machines are based on microwave linear accelerators. The majority of medical accelerators use frequencies in the S-band range. Having a compact accelerator allows for a wide range of treatments. The size and weight of the accelerator is substantially reduced if a higher frequency is used. X-band frequencies are suitable for such applications. The X-band accelerator technology has been used in high-energy as well as industrial applications. In the radiation therapy field, it is already implemented in some machines. The Mobetron [1], an Intra-Operative Radiation Therapy (IORT) treatment system is one example. Another example is the Stereotactic Radiosurgery machine, the CyberKnife [2]. The compactness of these machines required the use of an X-band accelerator. The basis for choosing the X-band technology in some of the medical machines is analysed. A review of the exiting medical applications is included. We also discuss the availability of other X-band components in the machine, including high-power RF sources. 1 X-BAND ACCELERATOR TECHNOLOGY X-band accelerator development has gained great momentum in the last ten years, motivated by the need for high-gradient accelerators for the future linear colliders in high-energy physics research [3]. Design of accelerator cells [4], manufacturing [5], and characterization technique [6] have been developed. In the industrial [7] and medical [1,2] fields, there are obvious advantages for using frequencies higher than the S-band range that is prevalent in these applications. The dominant factor is the compactness of the accelerator with the attendant reduction in weight. Consequently, the linac motion can be precisely controlled. By implementing the X-band technology, one can use a shorter length for a given power to achieve a certain electron beam energy. The reasons for this are two-fold. First, the shunt impedance per unit length is higher than that of S-band. Second, the maximum permissible electric field strength is also higher. The frequency dependence of these accelerator parameters is listed in Table 1. ___________________ Email: [email protected] Table 1: Frequency Dependence of Accelerator Parameters Parameter Frequency Dependence Shunt impedance per unit length (r) f Maximum permissible electric field strength f RF loss factor (Q) f 1/2 Power dissipation capability of accelerator structure f 1 Certainly there are some trade-off factors to be considered with the use of X-band. These include the lower Q and lower power dissipation capability of the accelerator structure, as indicated in Table 1. These can be overcome by careful design of the cooling system. In addition, a better surface finish and tighter dimension tolerances are required. 2 MEDICAL APPLICATIONS Currently, there are two different radiation therapy applications that use the X-band technology. 2.1 Inter-Operative Radiation Therapy The Inter-Operative Radiation Therapy (IORT) technique basically refers to the delivery of radiation during an operation using an electron beam. This technique of radiation therapy has been proven to increase the survival rates in certain cancer cases. Conventional radiation therapy machines must be housed in special shielded rooms, requiring the patient to be moved from the operating room. A mobile radiation therapy machine, the Mobetron [1], engineered and developed by Intraop, has met the need for a self-shielded machine. Since the unit is mobile, it can be easily positioned in an operating room to treat cancerous growths with good precision during surgery, thus, increasing the effectiveness of radiation doses to tumors while reducing the dose to surrounding healthy tissue. The design approach for the Mobetron exploits some of the unique advantages of using the X-band technology mentioned above. The Mobetron treatment head houses the X-band linac, the magnetron, the pulse transformer, RF loads, the circulator, and other RF circuitry. 0-7803-5573-3/99/$10.00@1999 IEEE. 2516 Proceedings of the 1999 Particle Accelerator Conference, New York, 1999