Removal of electron contamination in longitudinal field MRI-Linac Systems: A Monte Carlo Study

Purpose: The prototype inline MRI‐linac system has some advantages over perpendicular models including avoiding the electron return effect. One of the disadvantages of the inline approach is the increased skin dose, estimated to be 400–1000% of the dmax dose. The purpose of this work was to design a feasible method to reduce this skin dose to acceptable levels. Methods: Magnetic modeling of proposed MRI‐linac designs have been simulated with the inclusion of an optimized permanent magnet system to purge/deflect the electron contamination. The region of air above the phantom was also replaced with a helium bag (region of helium gas) and a beam scrapper below the deflector was added to collect deflected off‐axis contamination. Monte Carlo simulations were then performed including the accurate 3D magnetic field maps. Surface dosimetry was recorded to verify the changes to the skin doses. Results: Magnetic modelling showed that an optimized NdFeB permanent magnet system located outside the MRI coils (below the MLC's) can provide a strong enough region to purge/deflect a significant portion of the electron contamination from the x‐ray beam. The impact on the MRI uniformity is around 100 ppm and hence is correctable via active/passive shimming of the MRI. The helium region also significantly limits the production of contamination traveling towards the phantom surface. Entry doses near CAX are predicted to be similar to the 0 T case. Conclusions: Magnetic and Monte Carlo modeling were performed to estimate the effect that a permanent magnet purging system, beam scrapper, and helium bag would have on lowering the skin doses in an inline MRI‐Linac system. MRI non‐uniformities introduced by the deflector could be corrected, contamination is mostly purged or blocked, and the helium bag minimizes air‐generated contamination. As a result skin doses are comparable to having zero magnetic field