  The comprehension of tumor growth is a intriguing subject for scientists. New
researches has been constantly required to better understand the complexity of
this phenomenon. In this paper, we pursue a physical description that account
for some experimental facts involving avascular tumor growth. We have proposed
an explanation of some phenomenological (macroscopic) aspects of tumor, as the
spatial form and the way it growths, from a individual-level (microscopic)
formulation. The model proposed here is based on a simple principle:
competitive interaction between the cells dependent on their mutual distances.
As a result, we reproduce many empirical evidences observed in real tumors, as
exponential growth in their early stages followed by a power law growth. The
model also reproduces the fractal space distribution of tumor cells and the
universal behavior presented in animals and tumor growth, conform reported by
West, Guiot {\it et. al.}\cite{West2001,Guiot2003}. The results suggest that
the universal similarity between tumor and animal growth comes from the fact
that both are described by the same growth equation - the Bertalanffy-Richards
model - even they does not necessarily share the same biophysical properties.
