We study a model introduced by Perthame and Vauchelet [17] that describes the growth of a tumor governed by Brinkman's Law, which takes into account friction between the tumor cells. We adopt the viscosity solution approach to establish an optimal uniform convergence result of the tumor density as well as the pressure in the incompressible limit. The system lacks standard maximum principle, and thus modification of the usual approach is necessary.Proof. This follows directly from the definition of viscosity subsolution and supersolution.An important property of classical viscosity solution is stability -if there is a uniformly convergent sequence of viscosity solutions, then the limit is also a viscosity solution. The next proposition asserts that the notion of viscosity solutions we define also enjoys such a property.Proposition 2.4. Let H and H n be functions on R n × (0, T ) × R n for all n = 1, 2, ... such that H n → H in L 1 ((0, T ), C(K)) for any compact subset K of R n × (0, T ) × R n . Let Q ⊂ R n × (0, T ). Suppose u n ∈ C(Q) are subsolutions (respectively, supersolutions) of u t + H n (x, t, Du) = 0 in Q for all n.