Calculation of microdosimetric spectra based on TRAX and its application for modelling RBE using the MKM

Besides the absorbed dose, the Relative Biological Effectiveness (RBE) is the most important quantity in treatment planning with ion radiotherapy. The RBE depends on many different parameters, such as: the biological endpoint, the dose, the particle type and energy and the tissue under consideration. In consequence, the RBE will be different at any location in the treatment field. As the biological effect of the radiation should be homogenous in the target it is necessary to predict properly the RBE in each position. Due to the many factors on which the RBE depends, it arises the need of using biophysical modelling for its determination (since it is not feasible to experimentally measure the RBE for all possible parameter combinations). We will focus here on two models: • the Local Effect Model (LEM) [1,2], which has been implemented in treatment planning in the GSI pilot project for ion beam tumor therapy, and • the Microdosimetric Kinetic Model (MKM) from Hawkins [2] as implemented by Kase [6]. The objective of this work was to calculate the microdosimetric spectra, as required as input for the MKM, using the TRAX code [4] and to compare the corresponding biological effectiveness with the predictions of the LEM.