The impression of being locked up in the world of illness, hospitals and medicine is part of what adolescents experience while undergoing treatment for cancer. This impression is exacerbated when they receive high-dose chemotherapy and a stem cell transplant in protective isolation. Confinement combines with and exasperates all aspects of their experience of cancer, especially the perturbation of their relationship with their body (which becomes strange or unfamiliar) and with others (withdrawal, flight, excessive demands, anger), of their sense of identity, difficulties in formulating and expressing their ideas, fear of thinking. Their parents are also upset. In order to help these adolescents live through this phase of their treatment and this confinement without being destabilized and, subsequently, to rid themselves of its psychical repercussions, psychoanalysts need to be sufficiently aware of the reality of this experience so that they can work on the elements of which it is composed and not on a rough definition of what it is and what phantasms it evokes. We describe first the elements involved in these treatments then the landmarks which can guide the psychoanalyst in these particularly difficult and complex situations.