An Analysis of the Carcinogenesis Testing Experience of the National Cancer Institute

Abstract

Tabulations of the results of the initial approximately 200 NCI carcinogenesis bioassays conducted in mice and rats were published recently. However, no analysis of the results were provided. Such an analysis is the subject of this report. Ninety eight NCI bioassays were positive for carcinogenicity. Only three bioassays (<2%) were negative in both species and the ratio of positive to negative bioassays was 33:1. The remaining 47% of the bioassays were considered to be suggestive or inconclusive and it is inappropriate to refer to the total positive rate as 50%. The liver was the predominant tumor site and the only tumors produced in either rodent species in 19% of the positive bioassays were in the livers of mice. The high ratio of positive to negative results and the use of the mouse liver as a discriminating target organ raise serious concerns about the carcinogenesis protocol and in the methods of evaluating the results. Several recommendations are offered which may be useful in the current reexamination of bioassay procedures.