Feasibility Study Of Bragg Peak Modulation With Porous Materials In Particle Therapy

Among the radiotherapy for tumors,ion beams have a strong relative biological effect because of their high linear energy transfer,which can destroy the doublestranded structure of DNA and treat radiation-insensitive tumors more effectively.The spot scanning technique can make the dose distribution of the target area conformal,but the sharp Bragg Peak(BP)has become an important issue affecting the treatment efficiency.For carbon ion beams,Ripple Filter(Ri Fi)is currently clinically used to broaden the BP.However,the carbon ion beam broadened by Ri Fi needs to be transported for a long distance to achieve fluence homogeneity,and it will also enlarge the isocenter spot size.In order to solve the problems brought by Ri Fi,international scholars have proposed a method of using porous materials(such as foams,sponges,lung substitutes,etc.)to broaden the BP,which can ensure the shortdistance fluence homogeneity.At the same beam range,the porous material can reduce the isocenter spot size to protect the normal tissues on both sides of the target.However,porous materials have not yet been clinically applied,and there are relatively few related studies,the modulation effect of porous material need to be thoroughly and comprehensively evaluated.In this paper,various parameters of carbon ion beam before and after modulation are taken as the research object,and the ability of porous materials to broaden BP and the feasibility of clinical application are evaluated by experimental and simulation optimization research methods.The main research contents include:Based on the treatment nozzle structure and carbon ion beam parameters of Shanghai Proton and Heavy Ion Center(SPHIC),a carbon ion beam model was established by using the Monte Carlo(MC)code FLUKA.It provided a foundation for the subsequent simulation work.The influence of beam optics asymmetry on the dose distribution in the target area was analyzed,which provided a reference for the selection of the beam parameters for the subsequent experimental work.Carbon granules were used to make the original porous material(OPM),and then experiment.With a similar broadening effect,beam parameters such as the distal falloff width(DFW)and the spot size(full width half maximum,FWHM)after modulated by using the porous material and 2D-Ri Fi were compared.By changing the beam incident positions on the porous material,differences in beam parameters were obtained.A binary voxel model(BVM)was used to describe the OPM and an original porous structure(OPS)established in FLUKA.Furthermore,an optimization method was proposed and a porous structure with plug-ins(PSP)was developed for broadening BP and counterpoising the sizeable beam DFW by using OPS.The performance of PSP was evaluated by comparing the properties such as broadening effect,lateral scattering,fluence homogeneity,and broadening stability,then the feasibility of its clinical application was verified.The related results are listed as follow:Compared with the clinically used 2D-RiFi and range shifter,spot size of the carbon ion beam modulated by porous material was reduced by 2.39 mm(35.3%),and the spot size of the proton beam was reduced by 5.11 mm(34.5%).With a similar broadening effect,the DFW of 234.05 Me V/u carbon ion beam increased by 2.04 mm(73.9%)compared with 2D-Ri Fi.The maximum difference of broadening effect between surrounding incident positions and the center position is 2.06 mm(40.9%).With a similar broadening effect,the DFW of the 234.05 MeV/u carbon ion beam modulated by PSP is reduced by 0.89 mm(46.1%)compared with OPS.With different energy levels,the maximum reduction of DFW can be up to 1.01 mm(54.3%).In terms of lateral scattering,the maximum reduction of spot size modulated by PSP was 1.72 mm.In terms of the fluence homogeneity,the flatness difference of 150 Me V/u carbon ion beam between by modulated PSP and the original value is 0.02%.In terms of the broadening stability,the differences of beam parameters can be guaranteed to be within 0.1 mm with 20 small voxels that the beam could pass through and the spot size that is not less than 6 mm.A structural optimization method based on OPS was proposed as the innovation of this paper.Compared to the OPS,the PSP developed by this method can greatly reduce the DFW of the broadened carbon ion beam.Through this study,the following conclusions can be drawn:1)With the same BP broadening effect and the same beam range,the isocenter spot size caused by the use of OPM is significantly smaller than that of Ri Fi.2)The DFW modulated by OPM is significantly larger than that of Ri Fi,and the stability of the broadening effect is relatively poor.3)With the same BP broadening effect and the same beam range,compared with Ri Fi,PSP can reduce the DFW after broadening,and significantly reduce the isocenter spot size,while ensuring the short-distance fluence homogeneity of the modulated carbon ion beam.PSP can ensure the stability of the broadening effect when the initial spot size is ≥ 6 mm.4)PSP shows better modulation capability of carbon ion beam than RiFi,and has the feasibility of clinical application.