Beam optics and isocenter property of SC200 proton therapy gantry

Studies on beam optics and the isocenter property in the gantry system for the SC200 (Superconducting Isochronous Cyclotron of 200 MeV in Hefei, China) are presented in this paper. The physical design of the isocentric gantry system is developed with the software TRANSPORT, which realizes a beam with a circular shape at the isocenter and a full width at half maximum of 4–10 mm. For stereotactic radiosurgery, the isocenter deviation of the gantry system should be less than 1 mm in diameter. In order to explore the property of the isocenter, an electromagnetic structure coupling analysis has been conducted given the self-gravity and the electromagnetic (EM) force of the magnets on the gantry beam line. The correlation between the isocenter property and the EM force has also been carefully studied. This paper puts forward two methods to obtain the isocenter deviation based on the characteristics of the nozzle installation structure and the calculation of the optical path after the nozzle, respectively. The results show that the maximum isocenter deviation is less than 0.33 mm with a safety factor of 1.5, and the deviation caused by the EM force is 0.05 mm. The latter result indicates that the impact of the EM force is negligible. This paper puts forward one possible way to realize real-time acquisition of the isocenter deviation in practical application. The gantry of SC200 is under construction at ASIPP.