Today, diagnosis of cancer and therapeutic choice imply strongly structured meeting between specialized practitioners. These complex and not standardized meetings are generally located at a same place and need a heavy preparation-time. In this context, we assume that efficient collaborative tools could help to reduce decision time and improve reliability of the chosen treatments. The European project Odysseus investigates how to design a Collaborative Decision Support Systems (CDSS) for surgical planning. We present here an activity analysis and the first outcomes of a participatory design method involving end users. Especially a new concept of Graphic User Interface (GUI) is proposed. It tries to make use of Virtual Reality technologies to overcome issues met with common collaborative tools.