The Local Effect Model - Principles and Applications

1. Abstract One major rationale for the application of heavy ion beams in tumor therapy is their increased relative biological effectiveness (RBE) in the Bragg peak region. For dose prescription, the increased effectiveness has to be taken into account in treatment planning. Hence, the complex dependencies of RBE on the dose level, biological endpoint, position in the field etc. require biophysical models, which have to fulfill two important criteria: simplicity and precision. High precision is required since steep gradients in the dose response curves are observed for most tumor and normal tissues; thus, even small uncertainties in the estimation of the biologically effective dose can transform into large uncertainties in the clinical outcome. In this contribution we will describe the so called 'Local Effect Model' (LEM), that has been successfully applied within treatment planning in the GSI pilot project for carbon ion tumor therapy and is now also implemented in a commercially available treatment planning system. The model is based on the knowledge of charged particle track structure in combination with the response of the cells or tissues under consideration to conventional photon radiation. Due to the high precision, the LEM seems to be adequate for the calculation of stochastic radiation effects, i.e. in the framework of radiation protection in addition to applications in tumor therapy.