Dosimetry Study Of Magnetic Resonance Guided Proton Therapy

Radiotherapy is an important way to treat tumors clinically.Compared with traditional photon therapy,proton therapy has the advantage of Bragg peak physical dose,which can provide a better dose distribution that conforms to the target area.However,the accuracy of treatment implementation is affected by uncertain factors such as patient positioning errors and breathing movement.Image guidance technology can reduce these uncertainties.Magnetic resonance imaging(MRI)has the advantages of high soft tissue resolution and no radiation dose.It is the best choice for image-guided radiotherapy.Magnetic resonance guided proton therapy(MRPT)is an important research direction to achieve precision proton therapy,but there are still many basic problems that need to be resolved.The interference of the MRI scanner on the proton therapy system and its beam delivery is one of the important problems.In the MRPT system,the proton beam delivery and dose deposition process will be affected by the MRI magnetic field.The current Monte Carlo method can accurately simulate the proton delivery process in the MRI imaging magnetic field(uniform magnetic field),but the calculation is time-consuming.Clinically,proton therapy is more inclined to pencil beam dose algorithms with computational efficiency advantages and reasonable accuracy.To apply magnetic resonance guided proton therapy to the clinic,it is necessary to develop a pencil beam algorithm under magnetic field conditions.The pencil beam algorithm is an analytical dose calculation method that does not need to simulate the delivery process of each proton like the Monte Carlo dose algorithm.Under non-magnetic field conditions,the trajectory of the proton beam is a straight line,the depth dose distribution can be calculated by analytical methods,and the lateral dose on the path is approximately a double Gaussian distribution,which can also be calculated analytically.The beam trajectory and dose distribution under magnetic field conditions will be much more complicated.This article aims to study the influence of a uniform magnetic field on the dose distribution and trajectory of the proton pencil beam,and propose a fast correction method for the Bragg peak location shift caused by the vertical magnetic field.The following are the results of the study:1.The dose distribution of the proton pencil beam in the water phantom under a uniform magnetic field is studied by Monte Carlo simulation.The study found that the parallel or perpendicular magnetic field did not change the characteristics of the Gaussian distribution of the lateral dose of the proton pencil beam,and the half-width parameter of the Gaussian distribution was not affected by the magnetic field.Secondly,the study found that the parallel magnetic field will not affect the overall dose distribution of the proton pencil beam,but the vertical magnetic field will cause the shift of the lateral dose.The shift near the Bragg peak is the most obvious.Studies have shown that the lateral shift of the Bragg peak has a linear relationship with the field strength and a cubic relationship with the initial energy of the proton.In addition,studies have shown that the magnetic field in air that in front of the water phantom will aggravate the shift of the dose.Generally,the higher the MRI field strength,the better the image resolution,but in a MRPT system with the vertical mode,the high field strength will mean a greater dose shift.In the case of significant dose shift,it is necessary to develop a method that can quickly and accurately correct the dose shift.2.The dose shift at the Bragg peak under the vertical magnetic field is the most obvious.This paper proposes the method of "angle correction + energy correction" to correct the Bragg peak location shift.The calculation of the correction parameters depends on the calculation of the beam trajectory,which is related to the energy loss.Based on the physical laws and geometric relationships,this paper derives the analytical calculation formula for the deflection trajectory of protons in the air,and the calculation error is within 0.1%(compared with the Monte Carlo simulation).In the water phantom,the energy loss of protons is complicated.Based on the information of the trajectory of the proton when it is incident along the beam line obtained by Monte Carlo simulation,the calculation method of the beam trajectory under different incident states is established according to the geometric relationship.Finally,according to the specific model and the calculation method of the trajectory,the calculation formula of the correction parameter is deduced.Based on the deflection data in the water phantom,the analytical formula can be used to directly calculate the correction parameters for different magnetic field ranges in air and irradiation directions,and the calculation can be completed within 1 second with the help of MATLAB programming.Finally,the correction effect was verified by Monte Carlo simulation,and the results showed that the position of the Bragg peak after correction was basically consistent with the expected position(the deviation was within 0.2mm).The "angle correction + energy correction" correction method proposed in this paper can effectively correct the Bragg peak location shift under the condition of the vertical magnetic field,and the calculation of the correction parameters is accurate and fast.The research on the dose distribution of the proton pencil beam and the calculation method of the deflection trajectory of the proton beam under the magnetic field condition can provide a reference for the development of the proton pencil beam dose algorithm under the magnetic field condition and the beam delivery of the MRPT system in future.