Although DNA damage is widely viewed as a critical target for the induction of cell killing by ionising radiation, the exact nature of DNA damage responsible for these effects is unknown. To address this issue, the probability of forming lethal damage by single proton tracks, derived from published survival data for Chinese hamster V79 cells irradiated by protons with energies from 0.57 to 5.01 MeV, has been compared to estimated yields of clustered DNA lesions and repair outcomes calculated with Monte Carlo models. The reported studies provide new information about the potential relationship between the induction and repair of clustered DNA damage and trends in the expected number of lethal events for protons with increasing linear energy transfer (LET). A good correlation was found between the number of lethal events in V79 cells and the induction of double-strand breaks (DSBs) consisting of three or more elementary DNA lesions. For the yields of other types of DNA damage, as well as point mutations formed through the misrepair of base damage and single-strand breaks, observed trends with increasing LET are not consistent with trends in the yields of lethal events. This observation suggests that the relative biological effectiveness (RBE) of protons of varying quality may be more closely related to the induction of complex DSBs rather than other forms of damage.

Institute of Physics Academy of Sciences of the Czech Republic Na Slovance 2 18221 Praha 8 Czech Republic

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Analytical formulas representing track-structure simulations on DNA damage induced by protons and light ions at radiotherapy-relevant energies