Exponential growth of cancer cells. (a) Variability in cancer due to an increase in risk with age of four individuals belonging to groups indicated by: H, high; L, low; P, predisposition; D, dose. The values for predisposition are: τ0 = 5 y (HP), τ0 = 20 y (LP). The effect of age and absorbed dose is reflected by the values: b= − 0.01 y−1 (HD), and b = − 0.005 y−1 (LD). (b) Exponential growth in cancer cells after crossing the risk-threshold illustrated in (a).