Table 1. .

Summary of LQ model: Contrary to common misconceptions, α and β are not dose specific coefficients for SSB and DSB DNA breaks. Many SSB and DSB, existing in close proximity, and before repair, are required to produce a lethal event (normally accompanied by the appearance of a lethal chromosomal aberration to cause cell death due to asymmetric segregation of genetic information at mitosis). Consequently, SSB and DSB are necessary precursors, but not sufficient requirements, for lethality. In cell survival experiments, α and β must reflect the probability of lethality with increasing dose and are best defined as: αis the coefficient of lethal events per cell per unit dose (units Gy−1) βis the coefficient of lethal events per cell per unit dose squared (units Gy−2) In this way, α and β probably reflect the number of lethal chromosomal events with dose and dose squared respectively. It follows that the probability of cell survival (where there are no lethal events), referred to as the Survival Fraction (SF), is obtained using Poisson statistics: ${\displaystyle SF=e^{-\left(expectednumberoflethalevents\right)}=e^{-d-b{d}^2}}$ For n fractions of dose d, then ${\displaystyle SF=e^{-n\left(d+b{d}^2\right)}}$. The LQ model can alternatively be expressed as: α = Σ (localised unrepaired DNA damage ⇒ lethal chromosomal injury events per cell)/dose β= Σ (localised unrepaired DNA damage ⇒ lethal chromosomal injury events per cell)/dose2 The predominant mode of cell killing at low dose is via α-mediated damage, but β-mediated damage dominates at high dose (Figure 1).

DSB, doublestrand; SSB, singlestrand.