Research On Fringe Field Effect And High-order Aberrations For Proton Therapy Gantry Beamline

As an advanced method for cancer radiotherapy,proton therapy has been attracting more and more attention due to its excellent dose distribution characteristics.United with several research institutions,Huazhong University of Science and Technology has been developing a proton therapy facility based on a superconducting cyclotron.The gantry beamline is an important part of the facility,and its optics will determine beam quality at the iso-center directly.The beamline optics can be designed through the simplified magnetic field model and the linear transfer matrix.However,the actual magnetic fringe field and the high-order terms of particle motion can not be precisely described in this approach.It may deviate the actual result from the design value,which increases the difficulty and uncertainty during beam commissioning.Meanwhile,the fringe field interference between neighboring magnets in a compact layout also brings challenges to the design of steerer magnets.Based on these,the fringe field effect and high-order aberrations of the proton therapy gantry beamline has been studied.According to the second-order beam optics,this paper proposed the FEA/ODE beam analysis method,which is based on the characteristics of the magnet system with a midplane symmetry and high-precision finite element electromagnetic simulation software.By using the MATLAB ODE toolbox and OPERA software,the method has been successfully applied to the verification of the present gantry beamline.The result suggests that although the beam envelope is acceptable,the beam at the iso-center is dispersive and asymmetrical.The major cause of this phenomenon is the magnetic fringe field,and the second-order aberrations have minor contribution.For the potential problems of present optics design,this paper proposed an optics optimization method based on COSY INFINITY code.The Enge function is used to fit the actual magnetic fringe field,and the gradient of quadrupoles is re-matched by multiobjective optimization without changing the system layout.It has been proven that the optimized beamline optics meets the design requirements,and the beam spot at the isocenter is close to a circular symmetrical structure,which improves the stability and reliability of the proton therapy.In addition,according to the requirements of the actual trajectory correction,this paper compared two kinds of steerer magnet schemes of “O” type and “H” type.By studying the interference effect of the fringe field between steerer magnets and neighboring magnets,it has been suggested that the “H” type steerer magnet has higher efficiency and more flexible position planning.After comprehensive consideration,the “H” type magnet is selected as the solution for the steerer magnet.At the same time,according to the actual coil winding,cooling and the requirements on the good field region,the original “H” magnet has been redesigned.The optimized steerer magnet was be adopted in the actual gantry beamline.