Model-based time extrapolation for quantal response studies

It is often desired to compare chemicals with respect to toxicity for purposes of priority setting in regulation. Long-term carcinogenicity studies are frequently used as the basic data for such exercises. When the results of these studies for different chemicals are compared, many confounders potentially arise. One confounder is that these studies may have been conducted for different study lengths. In this case, converting the results of these studies to a common "standard" time length would be of interest. We propose an adjustment to modify the results of a study with a particular study length to a standard time length. This adjustment is based on a simple stochastic model for carcinogenicity studies. We illustrate the application of this adjustment with an example of a chemical that has been studied by the National Toxicology Program.