Monte Carlo Simulation Of Dosimetry In Magnetic Resonance Guided Carbon-ion Radiotherapy

Proton/heavy ion radiotherapy emerged to overcome the deficiency of conventional radiotherapy,such as the inability to carve out the tumor dose,the insufficient protection of normal tissues,the inability to overcome tumor hypoxia,and the inability to distinguish tumor heterogeneity.The biggest difference between proton/heavy ion radiotherapy and traditional photon radiotherapy is that the proton/heavy ion beam will stop at a certain depth in the human body and release most of its energy,so as to achieve "targeted radiotherapy" to a certain extent.But in the actual treatment process,the location of the tumor may be change due to the patient’s respiratory,gastrointestinal peristalsis,swallow,muscle contraction and heart beat physiological movement.In addition,patients may also engage in some involuntary activities due to pain or nervous when they lie on the treatment bed.The physiological movements and involuntary activities of patients increase the uncertainty of the treatment area.As a result,the doctors have to put a certain boundary outside when drawing the target area,which increases the treatment volume.However,this may not be accepted for proton/heavy-ion radiotherapy.Therefore,it is more important for proton/heavy ion radiotherapy to obtain the real-time images of tumor and OARs(Organs at Risk)before and during treatment.Currently,a variety of imaging methods have been applied in image-guided radiotherapy,such as X-ray imaging,ultrasound imaging,optical imaging and magnetic resonance imaging.X-ray imaging causes additional ionizing radiation to the patient.The resolution of ultrasound images is low,and strong dependence of operators.While,the optical images can only reflect the surface change of patients.Thus,above three imaging methods are not suitable for real-time image guided radiotherapy.Magnetic resonance imaging(MRI)because of its high soft tissue resolution,without ionizing radiation and other advantages become the best candidates for image guided radiotherapy.At present,many institutions in the world have carried out the system development of integrated magnetic resonance imaging components and accelerator devices,and built commercially available MR-guided radiotherapy system.Considering the steep Bragg peak of particle beam,the MR-guided technique will bring greater clinical gain to the particle radiotherapy.However,unlike photons,carbon-ions are charged and will be deflected in the magnetic field due to Lorentz force,resulting in a big change in the dose distribution.This change cannot be ignored and cannot be inevitable.Only studying the dose distribution change of carbon-ion beam in the magnetic field,and minimizing or modifying the change as much as possible,can the MR-guided carbon-ion radiotherapy technique become possible.In this paper,the Monte Carlo simulation software-Gate v8.2 was used to simulate the dose distribution of carbon-ion beam in the magnetic field,based on the beam delivery system of the deep treatment terminal of the Institute of Modern Physics.(1)In the existing MR-guided radiotherapy system,there are two modes of magnetic field and beam direction: vertical and parallel.We first simulated the dose distribution of the carbon-ion beam in the water phantom in different magnetic field directions,and the results showed that the influence of parallel magnetic field to carbonion beam dose distribution was not obvious,but the dose distribution of the carbon-ion beam in the orthogonal magnetic field changed significantly.(2)Unlike proton radiotherapy,a large number of secondary particles are generated when carbon-ions enter the medium.The dose distribution of these secondary particles cannot be ignored when calculate the whole dose distribution.We used the "particleFilter" module in Gate v8.2 to analyze the dose distribution of various secondary particles generated by the carbon-ions in the water phantom in the magnetic field.The results showed that the trajectories of the secondary particles in the magnetic field were basically consistent with the C12 primary beam.(3)Particle radiotherapy has two scanning modes: uniform scanning and spot scanning.We simulated separately the dose distribution of the carbon-ion beam in the water phantom in the uniform scanning and spot scanning in the orthogonal magnetic field.(4)Finally,based on the movement of charged particles in a magnetic field,an equation for calculating the trajectory of the carbon-ion beam in the orthogonal magnetic field was obtained.In addition,based on the existing research,we innovatively proposed to correct the deflection of the Bragg peak position of the carbon ion beam due to the presence of the magnetic field by scanning the magnet device and gave the method of calculating the beam correction angle at the position of the scanning magnet.The work done in this article provided reference data and research direction for the treatment planning system of MR-guided carbon-ions radiotherapy devices,and provided guidance for carbon-ions radiotherapy in the presence of a magnetic field.The scanning magnet correction method mentioned in this article was also applicable to proton radiotherapy.