Monte Carlo Study On The Dosimetry Related Issues In Heavy Ion Therapy

Compared to conventional radiations such as X-and60Co-rays, the mainreasons for using heavy charged particles in radiotherapy are their favorable inverteddepth-dose profile (―Bragg curve‖) and high relative biological effectiveness (RBE),ranging from relatively low values in the plateau region to a significant enhancementin the Bragg peak. These favorable features make heavy-ion therapy developedrapidly all over the world.Nowadays two different beam delivery strategies applied in tumor therapy arethe passive beam shaping system employed at the HIMAC facility, Japan and theactive one developed at GSI, Germany. In2009, the Institute of Modern Physics,Chinese Acadamy of Sciences(IMP-CAS) conducted the clinical trial of heavy ionsradiotherapy based on the Heavy Ion Research Facility at Lanzhou(HIRFL) andCooling Storage Ring(CSR) adopting the passive beam delivery system. Due to theuniform irradiation field restricted to5cm×5cm in the isocenter of therapy terminal,tumors in patients with larger area need be devided into several smaller parts, that isfield patching technique. However it comes with serious problems such as dosehotspots. Study on the field patching issues is important to improve beam deliverytowards the planned target volume and thereby to increase the tumor control rate.There are two different methods for evaluating radiation doses in patients. Oneis the analytical dose algorithm including the pencil beam algorithm and the broadbeam one. Another is the Monte Carlo (MC) method by using particle transport codes.Due to the lower accuracy of dose calculation and limited applicable scope of theformer method, MC dose algorithm with advantages of high accuracy and easy realization of modelling process becomes a demand in radiation treatment planningsystem (TPS). In this thesis, the MC software package GATE/GEANT4was adoptedto build the simplified simulation system of IMP therapy terminal, after done a greatdeal of simulation test and compared with the experimental results acquired at IMP,good agreement with a maximum dose deviation of3.4%was observed, whichsatisfied the TPS clinical requirements. Using the GATE simulation parameters,dosimetry related issues in heavy ion therapy were studied.Firstly, the GATE/GEANT4code was adopted to simulate the physical dose,dose-averaged LET and biological effective dose distributions. Then MC simulationconcerning the carbon ion beams delivered by the passive beam shaping system atHIRFL has been realized.Secondly, the lateral dose distribution of matched fields within±1.0millimeterof patient positioning error in carbon ion therapy was simulated. The relationshipbetween the central dose of the matched fields and the field patching gap wasobtained. Furthermore, the lateral dose profile of field patching irradiation with5.0mm gap was simulated. Then a feasible method for solving the issue of dose hotspotsduring field patching under the condition of higher patient positioning accuracies(patient positioning error within±1.0millimeter) was proposed.Thirdly, the lateral dose profiles of5.0cm×5.0cm radiation field with1~6mmextensions were simulated. The relationship between the edge dose and the sidelength of field was obtained, then5.0mm extension was put forward to increase thedose at the edge of the target.Finally, owing to scattering equilibrium effect on the central dose of target, alarge amount of MC simulation work has been done concerning several parameterssuch as the size of irradiation field and collimator to find the best condition for dosecalibration before IMP clinical treatments. Based on the simulation results, the bestplace of dose calibration was found in the plateau of Bragg curves. The resultsobtained in this thesis provide a substantial basis for studying dosimetry related issuesin the carbon ion radiotherapy at HIRFL definitely.