Microstructure changes in radiochromic films due to magnetic field and radiation.

Purpose: To correlate the dose response and changes in microscopic structures of the radiochromic films exposed to the clinical magnetic field in the range 1.5-3 T with standard and flattening filter-free (FFF) photon beams.

Methods: The radiochromic film was cut into 5 × 5 cm sheets/samples from one batch. These samples were exposed to a 1.5-T and/or 3-T B-fields from an MRI scanner using an abdominal sequence for 7 min before and after irradiation with 6 MV and/or 6 MV FFF beams. Films were placed in a reference condition at 5 cm depth in a solid water phantom and exposed up to 20 Gy. The sample orientation was maintained the same during exposure, readout, and scanning electron microscopic (SEM) analysis. The samples were scanned with an Epson Expression 11000XL in a 48-bit RGB color mode at 300 dpi with red channel. Scanned images were processed in Image J and red channel mean intensity values were recorded. The samples were then coated with 6 nm gold and imaged by SEM Teneo (5 kV, 13 pA) under 2000, 2500, and 3000 magnifications for texture analysis.

Results: The changes in the microstructure of the films in magnetic fields (1.5- and 3.0-T) were dose dependent. The orientation and granular size of samples at higher doses were altered compared to the controls. Needle-shaped structures of the active layer were longer and aligned for samples exposed to higher doses and magnetic field. However, no significant changes in optical density due to the presence of a magnetic field pre/postirradiation up to 20 Gy were observed.

Conclusion: Fine structures of the film represent the polymerization characteristics that are affected by the radiation dose in the magnetic field. Upon exposure to radiation, diacetylene monomers undergo polymerization that forms longer chains with a temporal response. Even though this study did not notice any significant changes in optical density due to the presence of magnetic field, this should be studied in simultaneous application of the magnetic field during treatment in a dedicated MR-linac unit.