Research On The Octupole Scanning Magnet Applied To Proton Therapy

In recent years,more and more attention is given to proton therapy as one of the effective therapy methods for cancer with the increasing incidence of cancer and the trend of patients to be youger.Generally,two separate dipole magnets are used to deflect the proton beam in the scanning system,which is installed on the rotating gantry as a main part of the treatment nozzle.It will increase the holistic size of the gantry,which leads to high cost of the therapy facility.In order to get a miniaturized and compact proton therapy device,one multipole scanning magnet will be used to replace two separate dipole magnets originally.A rotating dipole field will be generated for the octupole magnet in the central aperture when a regular sinusoidal current is given in each pair of its opposing poles which is provided by four independent current sources.Based on the principle of the multipole scanning magnet,it was proven that the value of the ratating dipole field in good field region would remain unchanged when the excitation current changed,which showed that it was feasible to deflect the beam in two orthogonal directions by one multipole scanning magnet.Its good field region should be a round shape.For the sake of the device’s requirements for the scanning magnet,the design parameters of the magnet were given.The number of poles of the multipole magnet was chosen to be eight after some comparisons.The appropriate aperture size and the shape of pole tips were selected after some models were analyzed in Opera-2D.The uniformity of the magnetic field was in the required range of±1.2×10^-2 in the good field region.The uniformity of the integral field was in the range of±1×10^-2.The uniformity in the good field region had hardly changed during the process of magnetic field rotation.The influence of the number of coil turns on the quality of the magnetic field was compared when the total current of the coil was unchanged.Beam tracking was analyzed to observe the influence of the inhomogeneity of the magnetic field in the good field region on the shape of the beam envelope.The changes of the ellipse shape of the beam in the irradiation field were given when the value or the direction of the magnetic field was changed.The actual envelope of the beam on the 4×4 lattice in the 1/4 circle of the irradiation field was analyzed.The shape of the beam envelope in the real space was given and the level of the distortion was less than 1.6%.The dynamic magnetic field was also analyzed.The changes of the excitation current’s amplitude of four pairs of coils at two adjacent scanning points remained unchanged in spot scanning.The influences of the eddy current on the magnetic field response under three dynamic processes were analyzed and the changes of the magnetic field quality in the good field region during the dynamic process were given.It was observed that the quality of the transverse field and the integrated field in the good field region remained well after the change of the excitation current ended.At last,the process of designing the octupole scanning magnet coils was introduced in detail.The geometric parameters,electrical parameters and the water cooling scheme of magnet coils were calculated.