Relative biological effectiveness (RBE) as a function of the linear energy transfer (LET) for different types of lethal damage in mammalian cells and for DNA damage. ILD, irrepairable lethal damage, derived as the contribution to the linear parameter α of the LQ model that is not repaired after irradiation of cells, even if maintained in conditions optimal for repair. PLD, potentially lethal damage, derived as the contribution to the linear parameter α that after irradiation is repaired in conditions optimal for repair. STLD, single track lethal damage, derived as the linear parameter α in conditions in which PLD is not repaired. SLD, sublethal damage, derived from survival curves as the square root of the quadratic parameter β of the LQ model. DNA-DSB, RBE for double strand breaks in DNA., DNA-SSB, RBE for single strand breaks in DNA. (from Barendsen et al.) .