“…Most of them reconsider the model in [5] and possibly extend it, e.g., to account for vascular and avascular growth of multicellular tumor spheroids [6]. The settings involve reaction-diffusion equations for the densities of cancer cells and of normal cells, coupled with an equation for the concentration of extracellular protons, see also [7,8]. All of these models, however, are set on the macroscopic scale of cell populations, whereas the microscopic, subcellular level is known to significantly influence (and even control) the macroscale behavior, e.g., by the intracellular proton dynamics, as mentioned above.…”