  Cancer is a disease that takes millions of lives every year. Then, to propose
treatments, avoid recurrence, and improve the patient's life quality, we need
to analyze this disease from a biophysical perspective with a solid
mathematical formulation. In this paper we introduce a novel deterministic
model for the evolution of tumors under several conditions (untreated tumors
and treated tumors using chemotherapy). Our model is characterized by a
second-order differential equation, whose origin and interpretation are
presented by exploiting our understanding of fluid mechanics (via continuity
equations) and the theory of differential equations. Additionally, we show that
our model can fit various experimental data sets. Thus, we prove that our
nuanced and general model can describe accelerated growth, as well as
cytostatic and cytotoxic effects. All in all, our model opens up a new window
in the understanding of tumor evolution and represents a promising connection
between the macroscopic and microscopic descriptions of cancer.
