Fiber-optic, real-time dosimeter based on optically stimulated luminescence of [formula omitted] and KBr:Eu for potential use in the radiotherapy of cancer

Abstract

This thesis describes a single-fiber dosimetry system based on optically stimulated luminescence (OSL) of artificially grown single crystals of [formula omitted] and KBr:Eu, with potential application in the medical field, especially in radio oncology. Small fiber-shaped dosimeters with dimensions (diameter/length) on the order of 500 μm/5 mm are attached to one end of an optical fiber, resulting in fiber probes with diameters of less than 1 mm and lengths of up to 15 m. The opposite end of the fiber is connected to an OSL reader that contains a stimulation light source (laser) and a photomultiplier tube that is used for luminescence detection. During irradiation, an optomechanical shutter periodically allows laser light to be transmitted down the optical fiber, to stimulate the luminescence response from the dosimeter being irradiated at a remote location. The luminescence measured during each interval of laser stimulation is indicative of the radiation dose absorbed in the dosimeter since the previous stimulation. The integral absorbed dose is obtained via a summation procedure from the measured dose fractions. Several operating procedures and data processing algorithms were developed in order to increase the speed and accuracy of the measurements, and integrated into the software that controls automated operation of the OSL readers. Periodic modulation of the stimulation also allows the OSL signal to be discriminated from background fluorescence, and thus yields measurements that are unaffected by Čerenkov light (the so-called “stem effect”). Depending on the type of material used, the speed of the measurements, expressed as the time required to estimate an individual dose fraction, can be as short as 67 ms. Integral dose estimates from real-time OSL of [formula omitted] and KBr:Eu were obtained for water-phantom irradiations performed with medical teletherapy sources, and were found to agree within 3.7% and 2.8%, respectively, with reference measurements from ionization chambers. We also investigated the possibility of using the fiber OSL readers for other purposes, such as two-dimensional mapping of dose distributions on compacted layers of [formula omitted] grains, and detection of low-level doses, for possible application in Homeland Security projects. © 2004, American Association of Physicists in Medicine. All rights reserved.