Accuracy of Megavolt Radiation Dosimetry using Thermoluminescent Lithium Fluoride

Abstract

The relative light output per Gy in polystyrene for roentgen beams of 6 and 42 MV and electrons between 2.2 and 34.5 MeV relative to 60Co gamma radiation is reported for different kinds of LiF dosemeters. The distribution of the absorbed dose inside a 0.25 and 0.4 mm thick LiF-teflon disc surrounded by polystyrene and irradiated with 60Co, 42 MV roentgen radiation and 39 MeV electrons was measured using 0.01 and 0.02 mm thick Lif-teflon discs. The measurements show that the absorbed dose distribution in the dosemeter depends on the energy of the radiation. When flat dosemeters were used, differences between the signals measured at the two orientations possible during read-out could easily amount to several per cent, and for this reason 0.4 mm and 0.5 mm LiF-Teflon discs were not trusted when the highest accuracy was required. The cavity theory by Burlin does not account for the phenomena caused by differences in electron scattering properties of the dosemeter and the phantom material. Some suggestions are presented for a different cavity theory for flat dosemeters dealing also with these phenomena. It describes the results to about the same degree of approximation as the Burlin theory, and fails to explain the observed energy dependence for electrons.

The quality dependence of LiF TLD in megavoltage photon beams: Monte Carlo simulation and experiments

“…This could be interpreted as independent verification of the IPSM (1990) dosimetry protocol and the associated calibration factors within the uncertainty of the TLD measurements and the Monte Carlo simulations. There is also agreement between our results and those of Rudén et al (1977) and Nyström et al (1994) to within 1%. Our results show that there is definitely a quality dependence factor for LiF TLD calibrated in 60 Co γ -rays and used for measurements in higher-energy photon beams.…”

“…A literature survey indicates that the values suggested for energy correction factors in megavoltage photon beams fall into three main groups. The first group show no need for an energy correction factor relative to 60 Co γ -rays (Bagne 1977) or a small energy correction less than 3% (Nyström et al 1994, Rudén andBengtsson 1977). The second group found a 4-8% decrease in response compared to 60 Co γ -rays but no quality dependence (Almond andMcCray 1970, Ogunleye andFregene 1981).…”