Variation in tissue correction factors for LiF, Al2O3 and Silicon Dosimeters as a function of tissue depth with comparison between intensity weighted mono-energetic photon and the poly-energetic photons used in brachytherapy and diagnostic radiology.

Poudel, Sashi

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Variation in tissue correction factors for LiF, Al2O3 and Silicon Dosimeters as a function of tissue depth with comparison between intensity weighted mono-energetic photon and the poly-energetic photons used in brachytherapy and diagnostic radiology.

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The MCNP6 radiation transport code was used to quantify changes in the absorbed dose tissue conversion factors for LiF, Al2O3, and silicon-based electronic dosimeters. While normally calibrated in-air and applied to all general geometric measurements, tissue conversion factors for each dosimeter were obtained at various depths in a simulated water phantom and compared against the standard in-air calibration method. In these experiments, a mono-energetic photon source was modeled at energies between 30 keV and 300 keV for a point-source placed at the center of a water phantom, a point-source placed at the surface of the phantom, and for a 10-cm radial field geometry. Again, mono-energetic photon source was modeled up to 1300 keV for a disk-source placed at the surface of the phantom and dosimetric calculations were obtained for water, LiF, Al2O3, and silicon at depths of 1 mm to 35 cm from the source. The dosimeter’s absorbed dose conversion factor was calculated as a ratio of the absorbed dose to water to that of the dosimeter measured at a specified phantom depth. The dosimeter’s calibration value also was obtained for both mono and polyenergetic source and the calibration value from poly-energetic source was compared with the intensity weighted average calibration value from mono-energetic photon. The calculated changes in the tissue conversion factors are significant because the American Association of Physicists in Medicine (AAPM) recommend that measurements of a brachytherapy or diagnostic source be made with an overall uncertainity of 5% or better. Yet, based on results, the absorbed dose tissue conversion factor for a LiF dosimeter was found to deviate from its calibration value by up to 9%, an Al2O3 dosimeter by 43%, and a silicon dosimeter by 61%. These uncertainties are in addition to the normal measurement uncertainties. By applying these tissue correction factors, these data may be used to meet the AAPM measurement requirements for mono-energetic and poly-energetic sources at measurement depths up to 35 cm under the irradiation geometries investigated herein.

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