The use of LiF (TLD-100) as an out-of-field dosimeter

Abstract

TLD-100 is a commonly used dosimeter, but it responds to neutrons that are produced during high-energy therapies. As a result, TLD-100 measurements outside of the treatment field are suspect when high-energy radiation is used. Although there are alternatives such as TLD-700 that do not respond to neutrons, such specialty dosimeters are expensive and not routinely used in most clinics. In the current study, we examined the accuracy of TLD-100 in measuring the out-of-field photon dose as a function of treatment energy.TLD-100 was irradiated outside of the treatment field around medical accelerators operated at 6 MV, 10 MV, 15 MV, and 18 MV to determine its accuracy compared to TLD-700. In an effort to eliminate the response of TLD-100 to neutrons, the TLD capsules were encased in varying thicknesses of cadmium foil (from 0.25 mm to 0.75 mm) before being irradiated at 18 MV. TLD-100 was found to be accurate out-of-field at 6 MV and 10 MV, but it over-responded substantially at 15 MV and 18 MV (by up to 1063% relative to TLD-700). By wrapping the TLD-100 in up to 0.75 mm of cadmium, it was possible to drastically reduce (down to 39% on average) the over-response of TLD-100;  however, we were unable to remove the over-response entirely. Although TLD-100 is well suited for measuring the dose out-of-field at energies as high as 10 MV, at higher energies (15 MV or greater), this dosimeter over-responds substantially and should not be used. Although encasing the TLD in cadmium minimized this over-response to a degree, it was not reduced sufficiently to make TLD-100 viable for measuring the out-of-field dose at high treatment energies.