Since 2001 a superconducting cyclotron for proton therapy has been designed in collaboration with the National Superconducting Cyclotron Laboratory (NSCL) at ACCEL Instruments GmbH [1]. The design is based on a NSCL proposal originating from 1993 [2]. In the recent years two cyclotrons of this type were built, tested as far as magnetic and cryogenic properties are concerned and installed at the customer’s premises. In this paper we present design features, operating parameters and results of quality assessment tests as well as important issues taken into consideration when building such a machine for medical application. INTRODUCTION In spring and summer 2004 ACCEL has delivered two superconducting cyclotrons dedicated for cancer treatment with protons. Based on a conceptual design of NSCL these machines are the first of their kind being engineered and built in industry. The first cyclotron was installed at the Paul-Scherrer-Institut (PSI) in Switzerland in spring 2004. As an essential part of the PSI project PROSCAN it will substitute an existing machine in order to intensify the activities in the field of cancer treatment. The second cyclotron was installed at the Rinecker Proton Therapy Center (RPTC) in Munich, Germany, and is now in the final assembly phase. It is part of a complete proton therapy system build by ACCEL. BASIC DESIGN CONCEPT Important machine parameters Important parameters of the 250 MeV proton cyclotron are listed in table 1. Starting from the NSCL proposal ACCEL has refined and finalized the design in close collaboration with NSCL. In particular the cryogenics had to be adapted to the needs of medical environment. Furthermore ACCEL was supported by PSI and KVI during the design phase. The major design goals are defined by the requirements of the medical application: • Fixed proton energy of 250 MeV (corresponding to approx. 40 cm range in water) • Maximum possible beam current of 500 nA • For beam scanning (used at PROSCAN and RPTC instead of scattering): stable beam current and fast intensity modulation • High availability / up-time • Fast and simple maintenance • Low activation • Closed cycle liquid He system • Standard industrial cryo coolers Fig. 1 shows a cut view on the main parts of the cyclotron. The most apparent part of the cyclotron is the pill box design of the iron yoke consisting of the flux return yoke rings and the motorized upper and lower pole cap. The rf-structures consist of 4 dees (two galvanically and two capacitively coupled satellites), four stems and a coupler. The system is driven by a 150 kW rf-amplifier. Table 1: Key Parameters of the 250 MeV cyclotron