Research On The Radiation Dosimetry Of MRIgRT System Under Magnetic Field

The MRI-guided radiotherapy technology has the advantages of high soft tissue contrast,real-time imaging,and non-radiation,thus become a hot topic of radiotherapy research at present.The strong magnetic fields associated with MRI scanner can influ-ence the trajectories of the secondary electrons via the Lorentz force,negatively affect-ing the dose distribution in the transport material.Thus,if the influence of the magnetic field on the charged particles is not been considered,the dose calculation of MRI-guided radiotherapy would be inaccurate.What's more,the calculation speed of dose under the magnetic field is also the key to the application of the Monte Carlo algorithm,which is the gold standard for dose calculation,in the clinical routine.Even though the cal-culation speed of traditional analytical dose calculation methods such as pencil beam algorithm as well as convolution/superposition algorithm are fast,these analytical meth-ods cannot reflect the influence of the above-mentioned magnetic field,so they cannot be directly used for dose correction under magnetic field and dose verification in clini-cal planning.Currently,the general dose calculation method under a magnetic field can only be based on the Monte Carlo algorithm.Although the international research on the development of MRI-guided radiotherapy equipment is in full swing,the development of independent dose verification software program for secondary check of MRI-guided radiotherapy and dose correction under a magnetic field are still rare.The purpose of this paper is to study the behavior of the dose under the magnetic field of MRIgRT and how to verify or correct it through independent dose verification tools.The specific research tasks include:(1)Constructing the photon-electron transport module under a constant magnetic field in Archer;(2)Studying the influence of the magnetic field correction factor of the ionization chamber dosimeter under different magnetic field strengths and directions as well as different beam qualities;(3)Modeling two virtual sources of ad-vanced MRIgRT accelerator heads(Elekta Unity and ViewRay MRIdian Linac)and integrating them into ArcherQA.Results:(1)Archer's calculation of the dose distri-bution under the magnetic field is very accurate,which is in good agreement with the general Monte Carlo software TOPAS.The relative percentage difference of doses in most areas is within 2%,but the calculation speed is about 260 times higher than that of TOPAS;(2)When the ionization chamber dosimeter is placed parallel to the magnetic field,the magnetic field has the least influence on the magnetic field correction factor kB;the magnetic field strength with the least influence on kB is 1 T;regardless of the parallel configuration or the perpendicular configuration,the magnetic field correction factor of the ionization chamber dosimeter PTW31010 is close to 1 at 1.5 T,that is,the correction of the magnetic field is negligible;(3)The established virtual source mod-els of the two MRIgRT accelerator heads(Elekta Unity and ViewRay MRIdian Linac)have preliminarily proved that they can be used for independent dose verification and meet the requirements of clinical use.The main innovations of this paper include:(1)Developed and verified the particle transport module in the magnetic field and added it to the GPU-accelerated Monte Carlo code Archer;(2)Proposed a method for calculat-ing the magnetic field correction factor using TOPAS,which effectively calibrates the accuracy of dose measurement using ionization chamber dosimeters under a magnetic field;(3)Independently developed virtual source models of MRIgRT equipment Elekta Unity and ViewRay MRIdian Linac.After been debugged and verified,ArcherQA can be used as an independent dose verification software program.