In this paper we aim to présent the methodology used in an Ergonomics research work to defïne the préparation for treatment in external radio therapy. This research work concems the analysis of the préparation for treat ment in external radiotherapy activity and the difficulties workers are experiencing. Lack of fluidity during this step and between the different actors was noted by the IRSN (Institute for Radioprotection and Nuclear Safety) experts and the ASN (Nuclear Safety Authority). IRSN assumes that those lacks of flu idity may hâve an impact on workers activity and patients' safety. ERSN formulated a thesis subject on the basis of this hypothesis called "Human activity and transverse performance, the case of préparation for treatment in external radio therapy".