In long term rodent carcinogenicity studies, the concurrent control group is the primary control group for data evaluation. However, there are instances in which the use of historical control data can help in the interpretation of experimental results, such as rare tumors and marginally increased tumor rates relative to concurrent controls. However, if historical control data are to be utilized in a formal testing framework, it is important that studies in the database be similar to the concurrent controls with respect to the factors known to influence tumor occurrence. These include dietary factors/body weight gain, gross necropsy and slide preparation procedures, histopathologic diagnosis, and study duration/survival of animals. Different types of control groups (eg, untreated and corn oil gavage) may also have different tumor rates. Time-related trends and inter-laboratory variability in tumor rates frequently observed in long term rodent studies are likely attributable to these factors. Examples are given to illustrate these sources of variability.
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