TH-E-BRB-10: Examination of General Cavity Theory for Lithium Fluoride TLDs in Bone and Lung Heterogeneities

Abstract

Purpose: To compare the measured and theoretical dose response of Lithium Fluoride Thermoluminescent Detectors (TLD-100) of different thickness in bone and lung heterogeneities. Methods: After determining individual TLD sensitivities, a total of 45 TLD chips (3.2×3.2 mm and depths of 0.15mm -thin, 0.38mm -medium and 0.89mm -thick TLDs) were placed in bone and lung slab phantoms and irradiated with a 6MV photon beam. According to the Burlin cavity theory, the proportion of the dose in the cavity that is due to secondary electrons generated in the medium is given by the parameter d which depends on β and g. Four different methods of calculating β and three formulas to calculate g were tested in this study which resulted in twelve different versions of the d parameter for both lung and bone. The mean stopping power ratios and mean mass energy absorption coefficients of the 6 MV photon spectrum in bone and lung heterogeneities were calculated with BEAMnrc Monte Carlo simulations. Then the ratio of dose in TLD to dose in medium (f) was calculated and compared against the measured values. Results: The Burlin cavity theory resulted in a good agreement with measurements for the TLDs in lung, where it overestimated the f factor by 1.9%, 2.2% and 3.0%, for thin, medium and thick TLDs respectively. However, the discrepancy between the calculated and measured f factors for bone were higher and the f factors were underestimated by 5.6%, 5.4% and 3.5% with the Burlin cavity theory for thin, medium and thick TLDs respectively. Conclusions: Better agreement of the theoretical and measured response of LiF TLDs was observed in lung than the bone heterogeneities. Furthermore all twelve versions of the Burlin cavity theory used in this study agreed to within 0.5% of each other in both lung and bone.