Impact of a Parallel Magnetic Field on Radiation Dose Beneath Thin Copper and Aluminum Foils

Purpose: The RF coils for magnetic resonance image guided radiotherapy (MRIgRT) may be constructed using thin and/or low-density conductors, along with thinner enclosure materials. This work measures the surface dose increases for lightweight conductors and enclosure materials in a magnetic field parallel to a 6 MV photon beam. Methods: Aluminum and copper foils (9–127 μm thick), as well as samples of polyimide (17 μm) and polyester (127 μm) films are positioned atop a polystyrene phantom. A parallel plate ion chamber embedded into the top of the phantom measures the surface dose in 6 MV photon beam. Measurements (% of dose at the depth of maximum dose) are performed with and without a parallel magnetic field (0.22T at magnet center). Results: In the presence of a magnetic field, the unobstructed surface dose is higher (31.9%Dmax versus 22.2%Dmax). The surface dose is found to increase linearly with thickness for thin (<25 μm) copper (0.339%Dmax μm−1) and aluminum (0.116%Dmax μm−1) foils. In the presence of a magnetic field the slope is lower (copper: 0.16%Dmax μm−1, aluminum: 0.06%Dmax μm−1). The effect of in-beam foils is reduced due to partial shielding of the surface from contaminant electrons. Copper causes a surface dose increase ≈3 times higher than aluminum of the same thickness, consistent with their relative electron density. Polyester film (127μm) increases the surface dose (to 35% Dmax with field) about as much as a gown (36% Dmax with field), while the increase with polyimide film (17μm) is less than 1% above the open field dose. Conclusions: Thin copper and aluminum conductors increase surface dose by an amount comparable to a hospital gown. Similarly, enclosure materials made of thin polyester or polyimide film increase surface dose by only a few %Dmax in excess of an unobstructed beam. Based on measurements in this study, in-beam, surface RF coils are feasible for MRIgRT systems.