Separation of scintillation and Cerenkov radiation in fiber optic dosimeters

Publication Year:
2018
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Repository URL:
http://ro.uow.edu.au/theses1/217
Author(s):
Madden, Levi
Publisher(s):
Centre for Medical Radiation Physics
Tags:
plastic scintillators; water equivalence; LINAC; megavoltage photon beam dosimetry; fiber optic dosimetry methods
thesis / dissertation description
A plastic scintillating fiber dosimeter has been developed that applies a novel temporal method to separate scintillation and Cerenkov radiation produced by pulsed external beam radiation therapy sources. Current single probe methods in megavoltage scintillation dosimetry apply optical filters that allow for characterisation of the Cerenkov radiation signal generated in irradiated optical fibers. These methods utilise an optical fiber with a split junction, allowing for simultaneous measurements of the single optical signal passed through different optical filters with separate photodetectors. Cross calibration of the two photodetectors responses is required; the accuracy of the method is dependent upon the calibration conditions being similar to those where the measurements are made. Cases where measurement conditions dont match calibration conditions include in vivo dosimetry and in vitro treatment planning dosimetry. The dosimetry system developed eliminates the requirement for optical filters and multiple photodetectors in single probe scintillation dosimetry provided the radiation source is pulsed. Water equivalence is a desired property for dosimeters as it allows for a simplified calculation of dose absorbed by water compared to that of water inequivalent dosimeters. The scintillator employed in the dosimetry system is required to have a long decay constant, however the scintillator implemented for its long decay constant, BC444 (Saint Gobain), was not known to be water equivalent. BC444 was studied through Burlin cavity theory and Monte Carlo simulations and was determined to be water equivalent for photons with energies between 200 keV and 20 MeV. The dose reproducibility of the dosimetry method was scored across a series of in vitro measurements at several depths and positions for depth dose curves and dose profiles respectively. The dose reproducibility per measurement was determined to be 2.4% relative response to 1 standard deviation. The novel scintillation dosimetry method demonstrated requires Cerenkov radiation response characterisation and analytical algorithm refinement to improve its dose response at low scintillator signal intensities. With future refinements, the plastic scintillating fiber dosimeter constructed is expected to be successful in high resolution dosimetry as an array and in vivo dosimetry as a water equivalent probe type dosimeter.