To better assess the potential biological consequences of diagnostic x-rays and selected γ -emitting radioisotopes used in brachytherapy, we used the PENELOPE Monte Carlo radiation transport code to estimate the spectrum of initial electrons produced by photons in single cells and in an irradiation geometry similar to those used in cell culture experiments. We then combined estimates of the initial spectrum of electrons from PENELOPE with DNA damage yields for monoenergetic electrons from the

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... st Monte Carlo damage simulation (MCDS). The predicted absolute yields (Gbp −1 Gy −1 ) and RBE values for single-strand break (SSB) and double-strand break (DSB) induction by 220 kVp x-rays are within 1% of the results from detailed track-structure simulations (Friedland et al 1999 Radiat. Environ. Biophys. 38 39). The measured RBE for DSB induction reported by Kühne et al (2005 Radiat. Res. 164 669) for γ -rays from 60 Co and for 29 kVp x-rays with a 50 µm Rh (mammography) filter are in excellent agreement (1.15 versus 1.16). DSB yields predicted by the MCDS also agree to within 7% with the absolute DSB yields reported by de Lara et al (2001 Radiat. Res. 155 440) and Botchway et al (1997 Radiat. Res. 148 317) for the irradiation of V79 cells by low energy (<2 keV) characteristic x-rays. The predicted RBE for DSB induction by γ -rays from bare 169 Yb and 131 Cs to 60 Co are 1.06 and 1.14, respectively. Tabulated RBE values for the single-cell and monolayer cell culture geometries differ by at most 15%. The proposed methodology is computationally efficient and may also be useful for the prediction of damage yields for mixtures of other types of charged particles, such as those found in proton therapy, space applications or internal dosimetry.