Room: Exhibit Hall | Forum 5
Purpose: Magnetic Resonance Imaging guided X-ray radiotherapy (MRIXT) offers the prospect of real-time tumor tracking at high soft-tissue contrast. However, a challenge in MRIXT is managing the Electron Return Effect (ERE) by which secondary electrons cause potentially dangerous enhancements in surface dose. The aim of this study is to characterize the dependence of the dose enhancement due to photon ERE on the magnetic field strength.
Methods: Monte Carlo simulations were performed to obtain depth-dose profiles for 6 MV and 10 MV ideal pencil beams traversing a 50x50x50 cm3 water phantom in the presence of a transverse magnetic field. At the inhomogenous air-water and water-air interfaces, the percentage dose increases (PDIs) over the zero-field values were then calculated for field strengths between 0 and 9 T.
Results: The results show that for both beams the dose is enhanced at both the entrance air-water and the exit water-air interfaces. At the exit interface, the PDI rises with magnetic field strength in the range of 0 to 0.5 T to reach a maximum of 85% and 95% for the 6 MV and 10 MV beams, respectively. In the 0.5 to 9 T range, however, the exit PDI falls approximately exponentially to a minimum of 16% for both beams (Figure 2). At the entrance interface, there is a monotonic increase in the dose which at 9 T is greater than the maximum zero-field value by 8% and 5% for the 10 MV and 6 MV beams, respectively.
Conclusion: The ERE has been observed as a function of magnetic field for photon beams. The results show that the dose enhancement at the exit surface may be effectively suppressed in high transverse magnetic fields (> 0.5 T). However, this has to be balanced against significant dose enhancements at the entrance surface.