Purpose: To understand the microstructure of film responsible for dosimetric differences when magnetic field and radiation are simultaneous used.
Methods: Radiochromic films (EBT3) from same batch were cut into 5x5 cm2 sheets and exposed to deliver 2 Gy and 16 Gy doses in a solid water phantom in the absence and presence of the magnetic field (0.35 T from MRIdian MR-linac) using 1.66x1.66cm2 field size. Orientation of the films was consistent throughout the measurements. Films were also imaged post-exposure to the known dose for 172 seconds using TRUFI MR-image sequence used clinically. Films were placed at 90 cm SAD at the depth of 1.5 cm. Within 48 hours of exposure films were scanned using Epson 1000 scanner. Additionally, small rectangular samples were cut from each film to perform Scanning Electron Microscopy (SEM) analysis. The samples were mounted on SEM stubs with carbon tape. After removing the top polyester laminate with tweezers, the stubs were coated with an Au-Pd layer of around 50 nm in thickness. The coated samples were characterized with a Tescan XEIA FEG SEM (Maryland NanoCenter, University of Maryland) at an accelerating voltage of 5 kV and 0.18 nA to acquire microstructure changes.
Results: Dose measured using RIT software with and without magnetic field was 35% and 21% different for 2 Gy and 16 Gy respectively. An extra 10% difference was recorded for film exposed in absence of magnetic field vs film exposed in presence of magnetic field and imaged for 172 second. Difference in particle orientation and background noise was visually noted from SEM sample. The microstructure changes with magnetic field are responsible for the differences in dosimetry characteristics of EBT film.
Conclusion: Magnetic field does change the microstructure of the film analyzed through SEM that impact the dosimetry in MR-Linac.
Not Applicable / None Entered.