In this paper, chemotherapy of cancer is studied.The model chosen to simulate the behaviour of the cancer isbased on the normal and the tumor cells evolution. Aggressivechemotherapy leading to tumor contraction must respect theconstraint on the patient health, which is the lower admissiblebound of the normal cells. The properties of positively invariantset are employed. The domains of attraction, representing allthe initial states of patients which could be brought back tothe healthy state by respecting the constraint on the patienthealth, are determined using set theory tools. The domains ofattraction corresponding to different chemotherapy profiles canbe characterized with this method.