Research On Key Techniques Of The Beamline Optics For A Proton Therapy Facility

As a kind of classic cancer treatment method,radiotherapy has a history of more than100 years.Photons and electrons are most commonly used in radiotherapy;however,they have limitations for deep-seated tumors due to the exponential attenuation of their dose distribution.Protons has a unique depth-dose distribution with the well-known 'Bragg peak' located at the end of the radiation range,which is related to the proton energy.Hence,protons are more preferable for most types of tumors due to accurate local dose control and minimum damage to the healthy tissues surrounding at the target tumor.The R&D of proton therapy system requires frontier technologies covering particle accelerators,medical physics,automatic control and mechanical engineering.Proton therapy facilities based on cyclotrons can deliver continuous wave(CW)proton beam with smaller footprint.Due to the fixed beam energy extracted from the cyclotron,an energy selection system(ESS)should be placed after the cyclotron to modulate the beam energy for treatment of various depth tumors.Some considerations for the beamline design are:(1)ESS should meet the clinical specifications for energy range,beam emittance and momentum spread.(2)The control of the beam loss and the layout of correcting magnets and beam diagnostics should be considered.(3)Complete beam transmission calculation is necessary for ESS,and intensity modulation should be adopted to decrease the beam current in high energy.(4)The types and numbers of the magnets in the beamline should be optimized for cost-effective consideration.Based on these issues,the main research contents of this dissertation include:(1)The optics of the gantry beamline and the periodic beamline are discussed and designed.Combined with collimator positioned at the entrance of the gantry beamline,the point-to-point 1:1 image optics from the coupling point(CP)to the iso-center can significantly increase the stability of the beam size at the iso-center with different rotating angles.(2)The main function of ESS is to realize the energy modulation while maintaining the beam emittance and momentum spread due to clinical specifications.Designs of the collimator set and the energy selection slit are performed with beam tracking and transmissions calculation in the dissertation.Finally,within the 70-240 MeV energy modulating range,the beam emittance can be chosen with 5? mm-mrad,7? mm-mrad,10? mm-mrad and momentum spread can be chosen with 0.3%,0.5%.(3)For different energy,the transmission efficiency of ESS is different,and the maximum ratio between high energy(230MeV)and low energy(70MeV)is more than 200.To relieve this difference,the intensity modulation scheme by defocusing beam at high energies on the downstream collimator in the beamline is introduced.Finally,the intensity ration can be controlled within 10.We use 7? mm-mrad emittance as a case study to demonstrate the intensity modulation scheme.(4)In the dissertation,we reported the physical design result of the quadpole prototype magnet.Two-dimensional cross-section optimization and three-dimensional pole-end chamfering are used to minimize the high order harmonic errors and the inhomogeneity of the magnetic field.Finally,both the field homogeneity and harmonic errors are controlled within specifications.