Investigation Of The Impact Of Magnetic Field On Multi-Particle Radiation Dose In MRIgRT Using Monte Carlo Software TOPAS

Image-guided radiotherapy(IGRT),including the newly developed MRIgRT,is one of the most effective ways for precise radiotherapy.However,MRIgRT requires radiation therapy planning systems to consider the impact of magnetic fields on the trajectory of charged particles,so that radiation dose in MRIgRT can be accurately calculated.Monte Carlo methods are the gold standard for radiation dose calculations because the interaction of particles in matter is fully considered even under the influence of magnetic field under MRIgRT.The objective of this research is to evaluate the impact of magnetic field on the dose distribution of multi-particle in MRIgRT.The tasks and methods include:First,a Monte Carlo software,TOPAS(TOol for PArticle Simulation),is used to simulate the dose distribution of multi-particle(photon,proton,carbon ion)in homogenous water tank with various magnetic field strengths.Second,the dose distribution of multi-particle in a heterogeneous medium(water-air-water phantom)is investigated.Finally,based on patient's CT image data,calculating dose distribution of multi-particle for patient.Results show that,firstly,the distance to dose maximum is reduced in the homogenous water tank irradiated by photons and a significant electron return effect occurs at the boundary of the medium in the water-air-water phantom within magnetic field.The dose increases significantly at the water-air boundaries while the dose decreases at the air-water interface.For the breast treatment plan,the skin dose increases within the magnetic field.While for the prostate,the magnetic field has very little effect on the dose.Secondly,when the energy is less than 150 MeV and the intensity of the magnetic field is less than 0.5 T,the shift is about 1 mm at the Bragg peak of proton,but the lateral shift of the Bragg peak can be about 4 mm in water phantom.For the combination of 150 MeV proton and 1.5 T magnetic field,though the shift of the field is only 1 mm at the Bragg peak,the lateral shift of the Bragg peak can be as large as 1 cm which is significant.At the interfaces of the water and air phantom,no visible effect of the magnetic field on the proton dose distribution can be observed.For patients diagnosed with nasopharyngeal carcinoma,the dose distribution is founded to be also deflected in the presence of a transverse magnetic field although the parallel scheme is simpler.Thirdly,the trend of carbon ion dose distribution is found to be consistent with the protons.However,the distribution is less affected within the magnetic field.In conclusion,in MRI guided multi-particle radiation therapy,we found that the magnetic field does affect the dose distribution and thus can use Monte Carlo methods to calculate and correct for such dose variations due to the magnetic field.In addition,for protons and carbon ions in the magnetic field,the dose distribution parallel schemes are found to be less affected by the magnetic field than transverse scheme.The research is one of the earliest effort of using Monte Carlo methods to investigate the impact of magnetic field on the particle dose distribution in detail from multiple perspectives,to analyze the dose feasibility of MRI guided radiation therapy,and to provide dosimetric references for the future development of MRI guided multi-particle radiotherapy.