Study On Dosimetry Issues Of Heavy Ion Medical Machine Based On Monte Carlo Method

The heavy ion therapy of tumor is known as the optimal radiotherapy modality for its great superiority including Bragg Peak,strong relative biological effectiveness(RBE)in comparison with conventional radiation therapy.In order to ensure the effectiveness of radiotherapy,the treatment planning system is used in heavy ion therapy to simulate the dose distribution in the Gross Tumor Volume(GTV)of patient before starting treatment.In this study,the MCNPX was applied to simulate the radiation energy loss of carbon ion in substance for its advantages in geometry processing and reaction cross section database.A series of research work were carried out based on HIMM which is conducted by Institute of Modern Physics(IMP),Chinese Academy of Sciences(CAS).The HIMM is the first heavy ion medical machine with independent intellectual property rights in China.The main research contents and conclusions of this dissertation are briefly summarized as below:1.A SIMD parallel computing platform of MCNPX based on MPI was developed in this work.To verify if the MCNPX code can be used in heavy ion therapy,the radiation energy loss in tissue-like media calculated with MCNPX was benchmarked with the experimental datas from GSI.The agreement between the simulation values of MCNPX and the GSI measurements was satisfactory over the Bragg curve.The deviation in plateau was ±1.3%-3.3% and the deviation in Bragg peak zone was less than ±3.8%.2.The RTIM(Resolution Transmission Interface Software for MCNPX)code has been developed to convert B-REP file into CSG file by proceeding boolean operation of the convex entities.Then material database has been established and allocated to the corresponding entity.The Compensator,Ripple Filter,Multileaf Collimator and Range Shifter of HIMM were modeled accurately for MCNPX.The volume deviation was on the small side which reflect the validity of the model.3.A series of experiments have been carried out on HIMM.The depth-dose distribution of particles which were modulated by Ripple Filter and Range Shifter in the tissue-like media water were calculated.The simulation results of MCNPX showed good agreement with the experiment datas in HIMM over the Bragg curve and the deviation was less than ±4.6%.4.An accurate digital human model based on CT phantoms was built and visualized accurately.The DCVM(Dose Calculation Viewer for MCNPX)code has been developed in this work in order to divide the CT images into matrix grid and establish the mapping relationship between every matrix grid and the elemental composition,mass density and geometric construction accordingly required in MCNPX.The patient phantom was precisely modeled at the greatest extent with ten million of grids.The VTK and Matplotlib were used respectively to realize the visualization of human model in 3D space and 2D space.5.The DCVM code was used to analyze the CT phantoms of a colon cancer patient.A treatment plan including the beam energy,Compensator,Ripple Filter,Multileaf Collimator and Range Shifter was developed in HIMM.A methodology has been proposed which is capable of simulating the carbon dose distribution in the patient CT phantoms on the basis of MCNPX and the established HIMM model.The results showed that there was almost no energy deposition in the black area which could be evacuated intestinal canal comparing with gray and offwhite area which were pathological human tissues and the air/tissue interface was conspicuous.The simulated results meet well with the theoretical expectation and that demonstrates the accuracy of dose distributions calculated by MCNPX in HIMM.