Variability in the rates of some common naturally occurring tumors in Fischer 344 rats and (C57BL/6N x C3H/HeN)F1 (B6C3F1) mice.

Variability inthe incidence rates of some common naturally occurring tumors for 72 inbred F344 rat and 54 (C57BL/6N x C3H/HeN)F1 (B6C3F1) mouse control groups used in carcinogenesis bioassays was evaluated. Significant heterogeneity of control rates was observed in at least two of six laboratories for lymphomas-leukemias, liver tumors, and pituitary tumors in the male rat and for pituitary tumors and endometrial stromal polyps in the female rat. Significant interlaboratory heterogeneity was observed for several tumor types in the F344 rat. In contrast, control incidence rates for tumors of the lung and liver and lymphomas-leukemias in B6C3F1 mice were relatively homogeneous within four of five laboratories. Significant interlaboratory heterogeneity was observed, however, for these mouse tumors. The causes of significant heterogeneity in naturally occurring tumor incidence rates within and among laboratories are unknown. Although the most appropriate and important comparison of an experimental group is with its matched control, there may be instances in which the historical control rates provide relevant data needed to clearly interpret carcinogenesis bioassay results. With the use of data from bioassays of 4-chloro-m-phenylenediamine and nitrilotriacetic acid, two examples are presented to demonstrate the usefulness of the historical control data.