Medical practices in oncology are expected to be more and more multidisciplinary, yet few articles studied how this may be concretely applied. This article is describing the functioning of a multidisciplinary specialized committee in a rare tumor. 219 medical cases during 26 meetings between 09/04/2003 and 14/02/2004 had been studied. Discussions had been observed, described and studied qualitatively and quantitatively. At the time of a national plan has been launched to impose obligatory the consultation of such committees before any treatment strategy, two lessons can be drawn from this case study. On the one hand, it shows that a multidisciplinary committee may be an organizational tool facilitating the emergence of collective decisions, since it may facilitate the formulation and the discussion of alternative options (72 cases out of 219). More generally, to take treatment decisions, members of the committee exchange arguments that deal mostly with clinical and psychological condition of the patient (50 cases out of 219), state-of-the-art knowledge (39 cases out of 219) and individual experience of the practitioners (33 cases out of 219). On the other hand, this article intends to underline that the creation of such committees cannot be considered as a sufficient condition to improve the quality of treatment decisions. Indeed, this case study shows that the quality of decisions does not only rely on the collective deliberation, but also on the individual and specific competencies of some members of the committee that other members admit.