One of the chief concerns of the medical staff of the Institut Gustave-Roussy is to find the right indications for treating every cancer site, and particularly cancer of the breast. The best treatment is the one which offers the patient the best chance of recovery, yet is at the same time the simplest and least mutilating [36]. It is our policy at the Institut, rather than to lay down a series of therapeutic measures which are applied systematically to all patients, to be selective, that is, to determine the best treatment for each individual patient. We make extensive use of prognostic factors, not merely for retrospective assessment when the treatment is completed, but very early on-during treatment or even before it is begun; thus, the treatment may be changed as a result, or simplified from the outset, when it is thought the outcome will be equally satisfactory. Our appreciation of the prognostic value of this or that clinical or biological factor is still far from complete, and we are continuing our studies in this direction.