Evidence for Power Law Tumor Growth and Implications for Cancer Radiotherapy

Understanding tumor growth dynamics is important for planning cancer therapy, especially in radiotherapy, where standard fractionation is determined based on the assumption that tumors grow exponentially. In this chapter, we explore the ramifications of an alternative, but equally plausible, assumption of tumor growth, namely power growth law, realized in linear increase in tumor diameter. We present a simple model for tumor growth, whose global dynamics demonstrate power law growth of the tumor, even under unlimited nutrient supply. For corroboration, we carry out and analyze one-, twoand three-dimensional tumor growth experiments both in vitro, in MCF-7 cells (breast cancer cell line) and in vivo, in mouse xenografts. In all studied cases, the unsaturated growth follows a power law growth function. Simulating radiotherapy under power law, Gompertz and exponential tumor growth, we show that the power law model predicts radically different outcomes for the conventionally used treatment. This demonstrates the importance of using the appropriate tumor growth model when calculating optimal dose fractionation schedule for radiotherapy.