Examination of general cavity theory for magnesium and titanium doped lithium fluoride (TLD-100) of varying thicknesses in bone and lung

Abstract

PurposeTo evaluate the accuracy of Burlin cavity theory for TLDs in bone and lung, the two most relevant heterogeneities in radiological physics. MethodsTheoretical calculations and Monte Carlo (MC) simulations of dose to TLD to dose to medium correction factor, (f)medTLD, were performed and compared in bone and lung. MC simulations included virtual irradiation of TLDs with varying thicknesses (0.015, 0.038, and 0.089 cm) in bone, lung, and water phantoms. Theoretical calculation of Burlin cavity theory requires calculation of fractional dose contribution from photon interactions (d) from mass effective attenuation coefficient (β) and average path length of electrons penetrating in the cavity from the wall (g). Different theoretical formulations of g and β were used to calculate 18 different values for d and (f)medTLD. Further, the impact of mean energy approximation used in theoretical calculations was evaluated using full spectrum MC simulations. ResultsWhile the values of d differed as much as by a factor of 2, (f)medTLD agreed well (SD = 0.1%) in water, bone and lung. The TLD thickness ranging 0.015–0.089 cm was not a significant factor (SD = 0.2%). Dose correction factors calculated using mean energy approximation agreed within the 2% with full spectrum MC simulations. Uncertainty associated with theoretical calculation of (f)medTLD was 7.2% compare to 0.5% with MC simulation. ConclusionThe (f)medTLD calculated with Burlin theory agreed well with MC results for 6 MV photon beam. Nevertheless, the difficulty and the ambiguity in the determination of β and g in a given medium and the energy spectrum under investigation limited the theoretical calculations and resulted in large uncertainty. This study suggests the use of MC for easy and accurate estimation of(f)medTLD, which is required in radiological applications to convert TLD measured dose to dose in medium.