  A fundamental model of tumor growth in the presence of cytotoxic
chemotherapeutic agents is formulated. The model allows to study the role of
the Norton-Simon hypothesis in the context of dose-dense chemotherapy.
Dose-dense protocols aim at reducing the period between courses of chemotherapy
from three weeks to two weeks, in order to avoid tumor regrowth between cycles.
We address the conditions under which these protocols might be more or less
beneficial in comparison to less dense settings, depending on the sensitivity
of the tumor cells to the cytotoxic drugs. The effects of varying other
parameters of the protocol, as for example the duration of each continuous drug
infusion, are also inspected. We believe that the present model might serve as
a foundation for the development of more sophisticated models for cancer
chemotherapy.
