Simulation And Design Of Dose Monitoring System In Proton Therapy System

The proton therapy facility based on superconducting cyclotron is a device used for proton radiation therapy for cancer,in which the dose monitoring system is a key part of achieving three-dimensional conformal dose radiation for proton beam.In this paper,according to the performance requirements of system,a real-time and on-line dose monitoring system is designed can detect low intensity and has a small impact on the beam.Based on the basic principle of dose detector and the theory of the gain and collection efficiency of the parallel plate ionization chamber,the design of the fundamental structure of the parallel plate ionization chamber is completed.The Monte Carlo software Geant4 is used to calculate the effect of parallel plate ionization chamber on beam.According to the beam current in the system and the gain of the parallel plate ionization chamber,the parameter design and principle simulation of the electronics of the dose monitoring system are performed.The designed electronics can detect proton beam currents at nA level.The conversion relationship between the electronic output frequency and the actual dose is given,and the preliminary design of the dose monitoring system is completed.As the beam detector destroys the vacuum environment of the nozzle,the smallscattering beam pipe is designed between the beam detectors in order to reduce the effect of the nozzle on the beam quality.According to the requirements of the system on the energy and transverse dimensions of the beam at the isocenter,the gas,sealing film,gas pressure and purity of the beam pipe are designed.The results show that the sigma of the beam spot of the 70 MeV proton beam at the isocenter is about 8mm,the energy loss is 1.76 MeV,while the sigma of the 250 MeV proton beam is within 3.5mm and the energy loss is 0.68 MeV.In the end,the difference between the actual dose distribution and the ideal design of the beam in the tumor area is analyzed with the change of the quality of the beam in nozzle to provide reference for the formulation of the treatment plan.