Characterizing magnetically focused contamination electrons by off-axis irradiation on an inline MRI-Linac

Purpose The aim of this study is to investigate off-axis irradiation on the Australian MRI-Linac using experiments and Monte Carlo simulations. Simulations are used to verify experimental measurements and to determine the minimum offset distance required to separate electron contamination from the photon field. Methods Dosimetric measurements were performed using a microDiamond detector, Gafchromic(R) EBT3 film, and MOSkin(TM). Three field sizes were investigated including 1.9 x 1.9, 5.8 x 5.8, and 9.7 x 9.6 cm(2). Each field was offset a maximum distance, approximately 10 cm, from the central magnetic axis (isocenter). Percentage depth doses (PDDs) were collected at a source-to-surface distance (SSD) of 1.8 m for fields collimated centrally and off-axis. PDD measurements were also acquired at isocenter for each off-axis field to measure electron contamination. Monte Carlo simulations were used to verify experimental measurements, determine the minimum field offset distance, and demonstrate the use of a spoiler to absorb electron contamination. Results Off-axis irradiation separates the majority of electron contamination from an x-ray beam and was found to significantly reduce in-field surface dose. For the 1.9 x 1.9, 5.8 x 5.8, and 9.7 x 9.6 cm(2) field, surface dose was reduced from 120.9% to 24.9%, 229.7% to 39.2%, and 355.3% to 47.3%, respectively. Monte Carlo simulations generally were within experimental error to MOSkin(TM) and microDiamond, and used to determine the minimum offset distance, 2.1 cm, from the field edge to isocenter. A water spoiler 2 cm thick was shown to reduce electron contamination dose to near zero. Conclusions Experimental and simulation data were acquired for a range of field sizes to investigate off-axis irradiation on an inline MRI-Linac. The skin sparing effect was observed with off-axis irradiation, a feature that cannot be achieved to the same extent with other methods, such as bolusing, for beams at isocenter.