Modeling the evolution of a tumoral multicellular spheroid as a two-fluid Bingham-like system

In this paper we propose a model for the evolution of a tumor spheroid assuming a structure in which the central necrotic region contains an inner liquid core surrounded by dead cells that keep some mechanical integrity. This partition is a consequence of assuming that a finite delay is required for the degradation of dead cells into liquid. The phenomenological assumption of constant local volume fraction of cells is also made. The above structure is coupled with a mechanical two-phase model that views the cell component as a Bingham-like fluid and the extracellular liquid as an inviscid fluid. By imposing the continuity of the normal stress throughout the whole spheroid, we can describe the spheroid evolution and characterize the possible steady state. Depending on the values of mechanical parameters, the model predicts either an evolution toward the steady state or an unbounded growth. An existence and uniqueness result has been proved under suitable assumptions, along with some qualitative properties of the solution