  The influence of random fluctuations on the recruitment of effector cells
towards a tumor is studied by means of a stochastic mathematical model.
Aggressively growing tumors are confronted against varying intensities of the
cell-mediated immune response for which chaotic and periodic oscillations
coexist together with stable tumor dynamics. A thorough parametric analysis of
the noise-induced transition from this oscillatory regime to complete tumor
dominance is carried out. A hysteresis phenomenon is uncovered, which
stabilizes the tumor at its carrying capacity and drives the healthy and the
immune cell populations to their extinction. Furthermore, it is shown that near
a crisis bifurcation such transitions occur under weak noise intensities.
Finally, the corresponding noise-induced chaos-order transformation is analyzed
and discussed in detail.
