  At present it is still quite difficult to match the vast knowledge on the
behavior of individual tumor cells with macroscopic measurements on clinical
tumors. On the modeling side, we already know how to deal with many molecular
pathways and cellular events, using systems of differential equations and other
modeling tools, and ideally, we should be able to extend such a mathematical
description up to the level of large tumor masses. An extended model should
thus help us forecast the behavior of large tumors from our basic knowledge of
microscopic processes. Unfortunately, the complexity of these processes makes
it very difficult -- probably impossible -- to develop comprehensive analytical
models. We try to bridge the gap with a simulation program which is based on
basic biochemical and biophysical processes -- thereby building an effective
computational model -- and in this paper we describe its structure, endeavoring
to make the description sufficiently detailed and yet understandable.
