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Safety culture has become an essential concept in the research on the reliability of risky socio-technical systems. In ergonomics, studying the safety culture means investigating ways of thinking, feeling and acting shared within a group of people involved in a common work activity and identifying the differences between a particular group and another one. Safety culture is often thought in a functionalist perspective: It is considered as a variable that can be isolated, analyzed and amended. Self-administered questionnaires are then frequently used to characterize safety culture. We propose a qualitative approach, the differential acceptability assessment method (D2A), based on a judgment of acceptability of actual deviations to prescribed practices. An empirical research is conducted in the field of radiotherapy. This medical specialty involves a transverse collective (i.e. a collective of professionals with different competences and tasks). The design and administration of the treatment require a series of different steps, involving four types of professionals who need to cooperate in order to deliver patient care safely. In this multi-profession context, we can expect a variety of working practices between the different subjects / trades. This does not mean that all practices are equally valid or that different radiotherapy professionals will judge them in the same way. We hypothesize that differences in judgment regarding deviations and in their underlying arguments are related to different safety cultures within the transverse collective. The objective of the research is to validate this hypothesis and to determine whether those profession-related safety cultures are an obstacle to the collective work that is necessary and fundamental in radiotherapy. The differential acceptability assessment method (D2A) used for this purpose. Assessing a safety culture through a questionnaire provides a quantitative measure of the level of safety culture. The acceptability differential method enables to highlight the diversity of judgments of acceptability of practices in a domain, and to interpret it in terms of differences in the safety cultures of the professionals involved.