Experimental characterization of magnetically focused electron contamination at the surface of a high-field inline MRI-linac.

ABSTRACT

PURPOSE: The fringe field of the Australian MRI-linac causes contaminant electrons to be focused along the central axis resulting in a high surface dose. This work aims to characterize this effect using Gafchromic film and high-resolution detectors, MOSkinTM and microDiamond. The secondary aim is to investigate the influence of the inline magnetic field on the relative dose response of these detectors. METHODS: The Australian MRI-linac has the unique feature that the linac is mounted on rails allowing for measurements to be performed at different magnetic field strengths while maintaining a constant source-to-surface distance (SSD). Percentage depth doses (PDD) were collected at SSD 1.82 m in a solid water phantom positioned in a low magnetic field region and then at isocenter of the MRI where the magnetic field is 1 T. Measurements for a range of field sizes were taken with the MOSkinTM , microDiamond, and Gafchromic® EBT3 film. The detectors' relative responses at 1 T were compared to the near 0 T PDD beyond the region of electron contamination, that is, 20 mm depth. The near surface measurements inside the MRI bore were compared among the different detectors. RESULTS: Skin dose in the MRI, as measured with the MOSkinTM , was 104.5% for 2.1 × 1.9 cm2 , 185.6% for 6.1 × 5.8 cm2 , 369.1% for 11.8 × 11.5 cm2 , and 711.1% for 23.5 × 23 cm2 . The detector measurements beyond the electron contamination region showed agreement between the relative response at 1 T and near 0 T. Film was in agreement with both detectors in this region further demonstrating their relative response is unaffected by the magnetic field. CONCLUSIONS: Experimental characterization of the high electron contamination at the surface was performed for a range of field sizes. The relative response of MOSkinTM and microDiamond detectors, beyond the electron contamination region, were confirmed to be unaffected by the 1-T inline magnetic field.