Impact of tissue specific parameters on the prediction of the biological effectiveness for treatment planning in ion beam therapy
Treatment planning in ion beam therapy requires a reliable estimation of the relative biological effectiveness (RBE) of the irradiated tissue. For the pilot project at GSI Helmholtzzentrum für Schwerionenforschung GmbH and at other European ion beam therapy centers RBE prediction is based on a bioph...
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|Zusammenfassung:||Treatment planning in ion beam therapy requires a reliable estimation of the relative biological effectiveness (RBE) of the irradiated tissue. For the pilot project at GSI Helmholtzzentrum für Schwerionenforschung GmbH and at other European ion beam therapy centers RBE prediction is based on a biophysical model, the Local Effect Model (LEM). The model version in use, LEM I, is optimized to give a reliable estimation of RBE in the target volume for carbon ion irradiation. However, systematic deviations are observed for the entrance channel of carbon ions and in general for lighter ions. Thus, the LEM has been continuously developed to improve accuracy. The recent version LEM IV has proven to better describe in-vitro cell experiments. Thus, for the clinical application of LEM IV it is of interest to analyze potential differences compared to LEM I under treatment-like conditions. The systematic analysis presented in this work is aiming at the comparison of RBE-weighted doses resulting from different approaches and model versions for protons and carbon ions. This will facilitate the assessment of consequences for clinical application and the interpretation of clinical results from different institutions. In the course of this thesis it has been shown that the RBE-weighted doses predicted on the basis of LEM IV for typical situations representing chordoma treatments differ on average by less than 10 % to those based on LEM I and thus also allow a consistent interpretation of the clinical results. At Japanese ion beam therapy centers the RBE is estimated using their clinical experience from neutron therapy in combination with in-vitro measurements for carbon ions (HIMAC approach). The methods presented in this work allow direct comparison of the HIMAC approach and the LEM and thus of the clinical results obtained at Japanese and European ion beam therapy centers. Furthermore, the sensitivity of the RBE on the model parameters was evaluated. Among all parameters the characterization of the photon dose-response curve has been found to be of particular importance for the determination of RBE. The application of the LEM IV for proton beams more correctly represents the experimentally observed increase of RBE towards the distal end of the irradiation field compared to the clinically considered constant value of 1.1. It further allowed a better systematic characterization of the increased effective range of proton beams that is a consequence of the RBE enhancement at the distal edge of the treatment field. The results of this work underline the importance of detailed RBE modeling for a long-term improvement of treatment planning in particle therapy and the better exploitation of advantages inherent to this radiation modality.|