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

The aim of this thesis is to examine the accuracy of Burlin general cavity theory in bone and lung heterogeneities when measuring dose using TLDs of various thicknesses. A practical framework to convert the dose to TLD to the dose in medium using a correction factor is developed and verified. Burlin cavity theory is verified in bone and lung, the most relevant heterogeneities in radiological physics. Theoretical calculations, experimental measurements and Monte Carlo simulations of (f)medium/TLDwere performed and compared for LiF: Mg,Ti (TLD-100) in bone and lung. Experimental setup included TLDs of three common thicknesses (0.015, 0.038 and 0.089 cm) that were irradiated in bone, lung, and water phantoms with 6 MV photon beam. (f)water/TLD is applied to convert the dose to TLD to the dose to medium, for which this study propose to use the Monte Carlo calculated values. Monte Carlo simulations of the same experiments were performed using BEAMnrc software. Theoretical calculations of Burlin general cavity theory were performed for 18 combinations of parameters including 6 different mass effective attenuation coefficient (β) and three different average path length of electrons across the cavity (g) which had been suggested in the previous studies. In the final step, (f)water/mediumfactors calculated with Burlin cavity theory were compared with MC simulation factors in bone and lung for an experimental setup generally being used during treatment planning system (TPS) commissioning in order to verify heterogeneity correction models. Theoretical f ((f)bone/TLD, ((f)lung/TLD) factors calculated for 18 combinations agreed well to each other and fall within 1% deviation to that of Monte Carlo analysis for both bone and lung (SD = 0.1%) heterogeneities. ((f)water/TLD factors of 0.833, 0.837 and 0.840 cm thick TLDs were used to convert the experimental TLD readings in bone and lung heterogeneities within the conversion framework being calculated with MC simulation. The f factor for bone was observed as decreases slightly for bone while it increases slightly for lung with increasing TLD thicknesses. In regard to the analyzes of results there were no statistically significant change in f factors with TLD thicknesses. The standard deviation differences between the MC simulations and theoretical calculations (f)water/medium factors for bone and lung using 0.089cm TLDs were 0.1% and 1.3%, respectively. The TLD measurements that were converted using theoretically calculated(f)water/medium factors have good agreement within 1% with the MC simulations for the experimental set up for both lung and bone. In order to achieve a systematic comparison in different materials Burlin Cavity theory was chosen as it is adaptable to bone and lung which haven't been studied yet or dose verification in medical applications. Instead of Burlin cavity theory for TLDs, we derived and verified a new framework to convert dose to medium. The results for bone and lung were in good agreement when compared with MC simulations and theoretical calculations. This study suggests that(f)water/medium simulated by MC can be used for dose to medium conversion especially in high Z materials such as bone. Burlin cavity theory for TLDs inserted in bone and lung, the two most relevant heterogeneity in medical physics, was found to be accurate within the uncertainties.