

Mammalian cell death after exposure to radiation after a dose D of radiation often follows the linearquadratic model (13).
Fraction of cells surviving = e(A*D + B*D^2)
The linearquadratic model was derived by Chadwick and Leenhouts (2). They proposed that the linear component (A*X) represents cell death due to a single lethal hit to the DNA, and that the quadratic (B*X2) component represents cell death that only happens with two hits. However, it is now clear that this mechanism is not correct, and the biological interpretation of the LQ parameters, A and B, is unclear (3).
Even though we don't know the biological basis for the model, it does a reasonable job of describing radiation induced cell death, except perhaps at very high radiation doses. Bodgi et. al. review the linearquadratic equation and some alternatives (3) and propose a biological model that corresponds to the linearquadratic model (4).
Sometimes the Y axis is plotted as the logarithm of fraction survival, rather than fraction survival itself.
Brenner, D. J. (2008). The linearquadratic model is an appropriate methodology for determining isoeffective doses at large doses per fraction, 18/4: 234–9. Elsevier.
Chadwick, K.H., Leenhouts, H.P., 1973. A molecular theory of cell survival. Phys. Med. Biol. 13, 78–87.
Bodgi, L., Canet, A., PujoMenjouet, L., Lesne, A., Victor, J.M., & Foray, N. (2016). Mathematical models of radiation action on living cells: From the target theory to the modern approaches. A historical and critical review. Journal of Theoretical Biology, 394: 93–101. DOI: 10.1016/j.jtbi.2016.01.018
Bodgi, L., & Foray, N. (2016). The nucleoshuttling of the ATM protein as a basis for a novel theory of radiation response: resolution of the linearquadratic model. International Journal of Radiation Biology, 92/3: 117–31. DOI: 10.3109/09553002.2016.1135260