The logic used in chemotherapy of breast cancer patients is intimately tied to growth patterns of breast cancer. When cancer is found in a patient, the tumor lies high on the growth phase of the Gompertz curve and is thus relatively slowly growing. Debulking the tumor by surgical removal and radiation puts any residual tumor in the smaller thus faster growing section of the Gompertz curve and makes it more chemosensitive. This reasoning is valid in animal models. Since human breast cancer is assumed to grow similarly, intensive chemotherapy is given shortly after surgery with the hope of eradicating all residual breast cancer cells.
Treatment is given until limited by toxicity and then stopped.
Then we hope for the best. Compelling though this model is, only modest improvements in survival rates have been made over the years. While it is often claimed that breast tumors grow in a Gompertzian manner, not all believe that. Such terms tempt one to imagine that every tumour has a specific equation of growth which may ultimately be understood in terms of biophysical parameters. Unpredictability', like the word 'spontaneous', is of course merely an admission that our knowledge is limited, but this itself should warn us against the attempt to imagine that all tumours conform to simple rules.