Fiber optic-based dosimeters composed of a scintillating sensor element connected to an un-doped transport optical fiber are interesting tools for radiation therapy dosimetry and quality assurance, due to their advantageous features including small physical size, mechanical flexibility, and being tissue-equivalent. With the introduction of emerging treatment modalities, especially with MRI-guidance available during radiation therapy, there is a need to fully investigate the influence of the magnetic field on the response of fiber optic dosimeters. In this work, we studied the Cherenkov light collected by transport optical fibers, manifested as a "stem effect" in fiber optic dosimeters. Two plastic fibers (core diameters of 1 and 2 mm) were irradiated with 6 MV photon beam generated using a conventional medical linear accelerator (no magnetic field) and an MRI-equipped linear accelerator with a 0.35 T magnetic field. The measurement setup was identical in both systems and the fibers received same amount of radiation dose. The fiber was placed between tissue-mimicking plastic layers at different depths and irradiated at a 10×10 cm2 field size. Spectroscopy was performed using a fiber-coupled spectrometer in 450 to 650 nm range with 2.5 nm resolution. We found that the amount of Cherenkov light collected by the fibers is increased in the presence of the magnetic field. The amount of this increase depends on the fiber's core size and depth of measurement.