Room: AAPM ePoster Library
Purpose: To investigate the influence of an external magnetic field on the Cherenkov signal collected by fiber optic dosimeters as a function of fiber size, field size, fiber orientation, and depth of measurement.
Methods: Two un-doped plastic transport fibers with core diameters of 2 mm and 1 mm were irradiated with 6 MV flattening filter free (FFF) photon using a MRIdianTM MR-Linac with B = 0.35 T magnetic field and a TrueBeamTM Linac (B = 0). The irradiation geometry was identical in both systems. The fiber was placed between solid water phantoms placed at surface, depths of 1 cm and 2 cm and 5 cm. The fibers were irradiated under two field sizes of 10×10 cm² and 15×15 cm². The 2 mm-core-fiber at a depth of 2 cm and field size of 10 ×10 cm² was rotated by 90°, 180°, and 270° relative to the B-field direction. A fiber coupled spectrometer was used for spectroscopy in 450 to 650 nm spectral range.
Results: The presence of the magnetic field increased the Cherenkov signal by approximately 20% and 15% at all the depths except at surface when the fiber was irradiated by 10×10 cm² and 15×15 cm² field sizes, respectively. The signal was decreased at the surface by about 30% under the magnetic field. The signal was increase by 25% and decreased by the same amount when the fiber was rotated by 270° and 90°, respectively. No difference was observed when the fiber was rotated by 180° compared to when it was at 0°.
Conclusion: The amount of Cherenkov signal measured by bare optical fibers changes in the presence of an external magnetic field. The amount of change depends on many factors such as the field size, depth of measurement, and the fiber orientation with respect to the magnetic field.