Ratcheting Up Cancer Potency Estimates

The current paradigm for cancer risk assessment in the United States (U.S.) typically requires selection of representative rodent bioassay dose response data for extrapolation to a single cancer potency estimate for humans. In the absence of extensive further information, the chosen bioassay result generally is taken to be that which gives the highest extrapolated result from the "most sensitive" species or strain. The estimated human cancer potency is thus derived from an upper-bound value on animal cancer potency that is technically similar to an extreme value statistic. Thus additional information from further bioassays can only lead to equal or larger cancer potency estimates. We here calculate the size of this effect using the collected results of a large number of bioassays. Since many standards are predicated on the value of the cancer potency, this effect is undesirable in producing a strong counter-incentive to performing further bioassays.