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Purpose: To present the derivation and application of a closed formalism for the impact of charged particle radiation on damage induced in DNA. The formalism is valid for all types of charged particles and due to its closed nature is suited to provide fast conversion of dose to DNA‐damage. As photon treatments can be reduced to energy deposition by electrons it is also possible to calculate damage for classical radiation treatments. Methods: A simple geometrical model allows to parameterize the impact of charged particles in terms of single and double strand breaks. The parameters of the model contain: double strand threshold (i.e. the maximal distance two single lesions can have to be categorized as a double strand break), the diameter of the DNA‐strand, and a scaling factor describing distance between ionizing collisions in terms of particle energy. The model is validated using microdosimetric Monte Carlo calculations for electrons and protons. Finally, example applications are constructed calculating Relative Biological Effectratios for proton therapy compared to a therapeutic 6MV beam. Results: For both protons and electrons the model fits the Monte Carlo calculations almost perfectly as assessed by a χ2‐test. The energy scaling factor seems to be the only difference between the modalities of protons and electrons. The RBE‐factor for a mono‐energetic proton beam varies depending on the depth and is of the order of 1.1 rising to almost 2 at the Bragg peak. Conclusions: We have shown that it is possible to provide a generalized expression parameterizing the DNA‐damage induced by different modalities, indicating that the change in biological effect is mainly governed by geometrical considerations rather than a biological impact. © 2013, American Association of Physicists in Medicine. All rights reserved.

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