Issues concerning the estimation of the TD50

The TD50 (or tumorigenic dose rate 50) is a generally accepted measure of the carcinogenic potency of a chemical in a particular strain of animal. This paper discusses error in the estimation of the TD50 caused by intercurrent mortality and error resulting from consideration of only significant TD50's. Using computer simulations, we found that treatment-related toxicity had only a small effect on estimating the TD50, with errors seldom exceeding 5%. The TD50 is sensitive to changes in tumor lethality with errors ranging to as high as 50%. Many of these errors were significantly different from zero and the results suggest that potency estimation could be improved by basing the estimates upon the tumor incidence rate rather than upon the tumor death rate when an estimate of tumor lethality is obtainable.