Analysis of uncertainties in organ dose in brachytherapy dose calculations

Dose calculations in brachytherapy treatment planning systems are performed in a homogeneous and infinite water environment using dose calculation formalism recommended by AAPM TG-43 report. Recently, model-based dose calculation algorithms developed as an alternative to TG-43 formalism have become available in the nowadays treatment planning systems for high-dose rate 192Ir brachytherapy applications. In contrast to TG-43 formalism, these algorithms take into account the actual patient sizes and tissue heterogeneities. Monte Carlo (MC) based algorithms, which are the basis for model-based dose calculation algorithms and which can calculate doses in a heterogeneous tissue media based on actual patient data, have not yet been included in clinical HDR 192Ir brachytherapy applications. However, in order to evaluate the differences between MBDCA and TG-43 based dose calculations, test case plans with MC methods are required. This thesis study consists of three stages in order to generate the data required for clinical use of MBDCA and to investigate the factors causing uncertainty in dose calculations. In the first stage, taking into account of the AAPM TG-229 report prepared for HDR source models, the current TG-43 dosimetric data sets were obtained with MC-based BrachyDose code system for 192Ir HDR sources and presented in comparison with the data sets of other studies in the literature. In this study, it was observed that the radial dose function values (for radial distances close to the source ) and dose rate constant were in close agreement with the other literature results. In the second step, detailed analyses were performed on TG-43 dosimetry parameters in order to accurately determine the factors affecting the patient dose estimation by taking into account the dosimetric data sets obtained in this study. When the results were examined, it was observed that the differences between the dose distributions obtained for distances less than 0.5 cm from the source and the calculated dose values for long distances from the source were dependent on the selection of dosimetric parameters. In the third step, in order to investigate the effect of medium materials on dose calculations, the dose values calculated using MC-based BrachyDose code system in an infinite water environment were compared with the calculated dose values considering the actual scattering conditions in a heterogeneous media. From the dose-volume graphs (DVH) obtained for water and tissue environments, a ~4% difference was observed between the D90 dose values, which included 90% of the planned target volume. When the dosimetric results obtained within the context of this thesis study are examined, it is seen that the actual organ doses taken by the patient will be different from the calculated dose values in the water when a tissue-equivalent medium is used.