Investigating the use of a solid water and copper electron filter for reducing skin dose in an inline MRI-linac

Skin dose measurements for the inline MRI-linac show a highly concentrated skin dose hot spot at the central axis of the beam. This is because the electron contamination is focused in the presence of the inline magnetic field, causing a significant entry surface (or skin) dose hot spot. The future clinical treatment will employ a 2 cm water electron filter to reduce electron contamination from the X-ray beam; however, this approach affects the overall dose distribution, and the skin dose remains significantly high.

To evaluate and compare the effectiveness of a copper electron filter (CuEF) and a water electron filter (WEF) in reducing the skin dose on the inline MRI-linac, using Monte Carlo simulations and experimental measurements.

The magnetic field maps, designed using a 3D magnetic field model for a 1 T MRI-linac in COMSOL Multiphysics, were subsequently integrated into the Geant4 Monte Carlo model of the Australian MRI-linac. The Geant4 geometry also included the WEF, CuEF and a 30  $\times$ 30  $\times$ 30 cm³ water phantom. Simulations were performed for field sizes 2.4  $\times$ 2.4 cm², 7.2  $\times$ 7.2 cm² and 12.4  $\times$ 12.4 cm². To compare the efficiency of a 2 cm water block in mitigating the electron contamination, simulations were performed with WEF positioned above the phantom surface. Central axis percentage depth dose and surface skin dose values at 70 $\mu m$ depth were measured using high-resolution scoring voxels. Additional simulations were performed by placing CuEF at varying distances from the MRI isocenter. The simulation results were further validated by the experimental measurements performed using the MOSkin™ detector by incorporating the physical prototypes of the CuEF and WEF in the pre-clinical setup of MRI-linac.

In the presence of a magnetic field, the skin dose values were measured to be 122.5% (2.4  $\times$ 2.4 cm²), 194.8% (7.2  $\times$ 7.2 cm²), and 260.4% (12.4  $\times$ 12.4 cm²) of the dose at d_max (1.5 cm depth) for near 0 T measurements. Adding the WEF 30 cm above the phantom surface reduces these skin dose values; however, the hot spot's intensity remains substantial, and the skin dose is higher than d_max. The CuEF was found to be more effective in reducing the skin dose hot spots across all three field sizes. Positioning the CuEF 10 cm above the phantom surface reduced the skin dose values by about 25%, 53%, and 64% for the field size 2.4  $\times$ 2.4 cm², 7.2  $\times$ 7.2 cm², and 12.4  $\times$ 12.4 cm², respectively.

Incorporating the CuEF above the phantom surface in the inline MRI-linac prototype notably reduces the intensity of the skin dose hot spot, proving to be a more effective solution than a standard 2 cm water block across all field sizes.