Development of dose calculation methods for brachytherapy treatment planning (in English)

The aim of the thesis is to develop methods for improving accuracy in dose calculations for photon brachytherapy treatment planning. This is achieved by separating the total dose into its primary and scatter components and through the use of three-dimensional integration methods to calculate the scatter dose. The collapsed-cone kernel-superposition algorithm, used clinically for treatment planning with external photon beams, was adapted for scatter-dose calculations in brachytherapy. A successive-scattering approach was developed to minimize the artifacts from the method's angular discretization of energy transport, which are otherwise a problem for the steep fluence gradients around brachytherapy sources. Methods for scaling kernels for heterogeneities, accounting for both photoelectric absorption and the emission of characteristic x rays, were derived. It is shown how data from a source-characterization formalism, that separates the total dose into its primary and scatter components, can support the collapsed cone algorithm with the input required to model clinical sources. With the methods developed in this work, the collapsed cone algorithm can be used for three-dimensional scatter dose calculations over the brachytherapy energy range, handle all heterogeneous materials and model clinical sources. An application in which this might be of particular importance is in designing individualized patient shields for intermediate energy isotopes such as 241Am and 169Yb. In this energy range (60-100 keV), thin foils of high atomic number effectively protect sensitive organs, however without three-dimensional scatter-dose calculations, errors as large as 10-20 % can occur within distances of therapeutic interest on the side targeted for treatment. Through use of the collapsed cone algorithm for scatter dose calculations, these dose reductions are predicted within 3% compared to the results of full-scale Monte Carlo simulations