A comparison of regulatory implications of traditional and exact two-stagedose-response models

We compare the regulatory implications of applying the traditional (lineari zed) and exact two-stage dose-response models to animal carcinogenic data. We analyze dose-response data from six studies, representing five different substances, and we determine the "goodness-of-fit" of each model as well a s the 95% confidence lower limit of the dose corresponding to a target exce ss risk of 10(-5) (the target risk dose TRD). For the two concave datasets, we find that the exact model gives a substantially better fit to the data than the traditional model, and that the exact model gives a TRD that is an order of magnitude lower than that given by the traditional model. In the other cases, the exact model gives a fit equivalent to or better than the t raditional model. We also show that although the exact two-stage model may exhibit dose-response concavity at moderate dose levels, it is always linea r or sublinear, and never supralinear, in the low-dose limit. Because regul atory concern is almost always confined to the low-dose region extrapolatio n, supralinear behavior seems not to be of regulatory concern in the exact two-stage model. Finally, we find that when performing this low-dose extrap olation in cases of dose-response concavity, extrapolating the model fit le ads to a more conservative TRD than taking a linear extrapolation from 10% excess risk. We conclude with a set of recommendations.