Among the proposed models, those based on Gompertz growth are frequently proposed [78]. It models the growth of a population consisting of a group of individuals of one or more similar species in the absence of migration and interaction with other species, by an S-shaped function C = C (t ) that is the solution of:
(7)
Within the context of tumor growth, C denotes the number of tumor cells at time t , C0 is the size of the tumor at the start of treatment, u(t) denotes the drug concentration at the site of action. C(t) is assumed to be continuous and differentiable [79]. Parameters α and β (measured in t-1 ), denote growth and decay rates, respectively, they characterize the evolution of different tumor types. During chemotherapy it is assumed that the basic growth kinetics of the tumor will be perturbed by the action of the cytotoxic drug u(t) .