ABSTRACT
Objective To evaluate the impact of an external magnetic field on the dose distribution and electronic disequilibrium region around a Bebig type 60Co HDR brachytherapy source and to judge the feasibility of applying MRI scanner during brachytherapy.Methods The source model was established based on the Monte Carlo package Geant4 software.The simulated geometries consisted of the 60Co source inside a water phantom of 10.0cm× 10.0cm× 10.0cm in size.The magnetic field strength of the 0T,1.5T and 3.0T was considered,respectively.The voxels with a size of 0.2 mm,0.5 mm and 0.5 mm were established along the x-,y-and z-axis.The influence of the magnetic field on the Kerma (kinetic energy released to matter) distribution and dose distribution within the 10.0mm region from the source center was evaluated.Furthermore,the ratio of the Kerma to dose as a function of the distance to the center source was acquired.Results The 1.5T magnetic field exerted no effect on the dose distribution adjacent to 60Co HDR brachytherapy source,whereas3.0T magnetic field caused significant increase in the dose distribution within r＜6 mm from the source center.The dose distribution was increased by 40％ at r=5.4 mm from the source center.The ratio of Kerma to dose was less than 1 within the region of 1.2 mm＜r＜6.0 mm,suggesting that 3.0T magnetic field can lead to electronic disequilibrium within a larger region from the source center.Conclusions For Bebig 60Co HDR brachytherapy source,it is safe and reliable to apply1.5T external magnetic field.Nevertheless,3.0T magnetic field can cause high dose risk.Consequently,safety assessment and verification should be delivered prior to clinical application.