  The speed and the versatility of today's computers open up new opportunities
to simulate complex biological systems. Here we review a computational approach
recently proposed by us to model large tumor cell populations and spheroids,
and we put forward general considerations that apply to any fine-grained
numerical model of tumors. We discuss ways to bypass computational limitations
and discuss our incremental approach, where each step is validated by
experimental observations on a quantitative basis. We present a few results on
the growth of tumor cells in closed and open environments and of tumor
spheroids. This study suggests new ways to explore the initial growth phase of
solid tumors and to optimize anti-tumor treatments.
