Technical note: Angular dependence in fiber optic dosimetry using YVO₄:Eu³⁺

Abstract

Background: Fiber optic dosimetry (FOD) has emerged as a useful technique that can be used in those cases when intracavitary, real time, high spatial resolution dose assessment is required. Among the several factors characterizing a dosimeter, angular response of FOD probes has to be assessed in order to consider possible clinical application. Purpose: The objective of this study was to characterize the angular response of a FOD probe based on a cylindrical shaped YVO 4 :Eu 3+ scintillator under irradiation with a 6 MV photon beam generated by a linear accelerator (LINAC). Methods: A FOD probe was irradiated inside a plastic phantom using a 6 MV LINAC photon beam at different azimuthal angles (0 • to 360 • ,15 • steps). Scintillation output was measured with a photomultiplier tube. Similar measurements were performed with a second FOD probe having an optical filter interposed between the scintillator and the fiber. Monte Carlo simulations using PENELOPE were carried out in order to interpret the observed results. Results: The FOD output was symmetrical with respect to the scintillator axis. For the unfiltered probe, the signal was maximum at rear incidence (0 • ) and steadily decreased down to its minimum value at frontal incidence (180 • ) having a signal ratio of 37%. The output of the filtered probe showed a plateau from 15 • up to 115 • . The signal was maximum at 60 • and minimum at 180 • having a signal ratio of 16%. Monte Carlo simulations predicted symmetry of the deposited dose about 0 • and 90 • ,which contrasts with experimental findings. Conclusions: Photoluminescence (PL) of the scintillator induced by the Cherenkov light increases the angular dependence.Radiation attenuation inside the scintillator and partial light collection of the scintillation yield by the optical fiber (OF) are responsible for asymmetrical response. Results from this study should be considered in order to minimize angular dependence in FOD.