Exact statistical tests for any carcinogenic effect in animal bioassays. II. Age-adjusted tests

Abstract

Statistical methods are discussed for application in animal carcinogenesis bioassays that test a general null hypothesis that response frequencies are independent of the treatment level for all tumor endpoints, sexes, and species considered in the bioassay, conditional on survival. These methods are similar to those proposed earlier by Farrar and Crump (1988, Fundam, Appl. Toxicol. 11, 652-663) which involve test statistics that are functions of p values from multiple standard statistical hypothesis tests, and evaluation of significance using randomization. Refinements include the introduction of an exact incidental tumor test analogous to the asymptotic test of Hoel and Walburg (1972, J. Natl. Cancer Inst. 49, 361-372), which takes into account the possibility that any differences among treatment groups in response rates result from differences in survival that is independent of tumors. Additional consideration is given to selection of test statistics, and a test is proposed that is sensitive specifically to cases where the same tumor endpoints are affected in multiple sexes and species. Applications of the procedures to results of National Toxicology Program (NTP) bioassays concerning decabromodiphenyl oxide and iodinated glycerol illustrate the sensitivity of tests based on different test statistics to distinct alternative hypotheses. When compared with the qualitative conclusions of the NTP regarding the strength of evidence for a carcinogenic effect in the same bioassays, the results presented suggest that the methods can be useful in clarifying otherwise equivocal evidence for carcinogenicity. Various difficulties are discussed relevant to the construction of exact combined fatal and incidental tumor tests analogous to the tests of Peto et al. (1980, IARC Monogr. Suppl. 2).