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Track structure effects in a study of cell killing in normal human skin fibroblasts

Purpose: To study track structure effects in cells irradiated by heavy ions, we have performed a model analysis of an extensive recently published data set of over 40 survival curves of normal human skin fibroblast cells irradiated in vitro by energetic carbon, neon, silicon and iron ions measured in track-segment conditions. Materials and methods: Having derived the required track-segment descriptions of the ion bombardments from the published data, we fitted four parameters of the cellular track structure theory (Katz model) to the whole data set. Results: Using track structure calculations with the best-fitted parameters, we demonstrate a systematic interpretation of this data set, highlighting effects specific to track structure. In particular, we model the dependence of relative biological effectiveness (RBE) and ‘single-particle’ and ‘extrapolated’ cross section on linear energy transfer (LET) or Z*2/β2 (where Z* is the effective charge and β is the relative velocity of the ion) and demonstrate the predictive capability of the model. Conclusions: Our interpretation of the data differs from that of Tsuruoka et al. We suggest that the biological effects of charged secondary particles generated in this experiment by degrading the energy of the primary ion beams using polymethyl methacrylate (PMMA) absorbers cannot be ignored.