Towards magnetic-field-modulated radiotherapy (MagMRT) with an MR-LINAC—a Monte Carlo study

The feasibility of magnetic-field-modulated radiotherapy (MagMRT) with an MR-LINAC was investigated by studying the effects of dose enhancement and reduction using a transverse magnetic field with a longitudinal gradient applied along a photon radiation beam. Geant4 simulation toolkit was used to perform Monte Carlo simulations on a water phantom with the energy spectrum of a 7 MV flattening-filter-free photon beam from an Elekta Unity system as the source of radiation. Linear magnetic field gradients with magnitudes ranged from 1 to 6 T cm and spatial extents of 1-3 cm were used to study the dependence of dose modulation on these two parameters. The effects of radiation field size and the ability of dose modulation through optimizing the waveform of magnetic field variation were also explored. Our results show that dose enhancement and reduction can be achieved by applying a transverse magnetic field with a longitudinal field gradient along a photon beam. The steeper the gradient, the more prominent is the effect. A dose enhancement of 33% and a dose reduction of 22% are found for a magnetic gradient of 6 T cm and -6 T cm respectively. The spatial extent of the dose modulation effect which is greater than 3% is found to be around 1-2 cm. Both the dose enhancement and reduction effects are independent of the radiation field sizes, but they exhibit different behaviors with the spatial extents of the gradient. Multiple locations of dose enhancement and reduction can be produced by modulating the waveform of the magnetic field variation along the radiation beam, demonstrating a vast degree of freedom in the modulation aspect of MagMRT. MagMRT is a conceptually feasible and promising new radiotherapy modulation technique along the direction of the radiation beam.