  We present a two-dimensional continuum model of tumor growth, which treats
the tissue as a composition of six distinct fluid phases; their dynamics are
governed by the equations of mass and momentum conservation. Our model divides
the cancer cells phase into two sub-phases depending on their maturity state.
The same approach is also applied for the vasculature phase, which is divided
into young sprouts (products of angiogenesis), and fully formed-mature vessels.
The remaining two phases correspond to healthy cells and extracellular material
(ECM). Furthermore, the model foresees the existence of nutrient chemical
species, which are transferred within the tissue through diffusion or supplied
by the vasculature (blood vessels). The model is numerically solved with the
Finite Elements Method and computations are performed with the commercial
software Comsol Multiphysics. The numerical simulations predict that mature
cancer cells are well separated from young cancer cells, which form a
protective shield for the growing tumor. We study the effect of different
mitosis and death rates for mature and young cancer cells on the tumor growth
rate, and predict accelerated rates when the mitosis rate of young cancer cells
is higher compared to mature cancer cells.
