A Method for Minimizing the Effects of Biologic Diversity on the Measurement of Tumor Growth Rates

When performing quantitative studies of tumor growth, one of the problems encountered is the inherent variability of biologic systems. Many factors influence the accurate measurement of tumor size, and tumor growth rates may vary widely within a single group of animals injected with identical amounts of the same tumor cells. These factors often cause difficulties when interpreting results and comparing experiments, and animal wastage often results. A simple model that uses exponential tumor growth kinetics for the comparison of tumor growth rates is proposed, which allows tumor growth rates within individual animals to be compared even when some animals within a group must be euthanatized earlier than planned because of rapid tumor growth. This model is applicable for tumors with simple kinetics over a particular range of tumor sizes and is independent of the day of tumor measurement. The use of similar control tumors in the same animal allows the direct comparison of experimental versus control tumors, and the effects of interanimal variation are eliminated. This method allows greater economy and efficiency in the use of animals, and allows the use of an appropriate endpoint for tumor size (maximum diameter of 15 mm). The model should not be used for tumors displaying complex kinetics such as initial growth followed by regression.