By studying a simple but realistic biophysical model of tumor growth in the presence
of a constant continuous chemotherapy, we show that if an extended Norton–Simon
hypothesis holds, the system may have multiple equilibria. Thus, the stochastic
bounded fluctuations that affect both the tumor carrying capacity and/or the drug
pharmacodynamics (and/or the drug pharmacokinetics) may cause the transition from a
small equilibrium to a far larger one, not compatible with the life of the host. In particular,
we mainly investigated the effects of fluctuations that involve parameters nonlinearly
affecting the deterministic model. We propose to frame the above phenomena as a new
and non-genetic kind of resistance to chemotherapy.
