Neutron Therapy: An Experimental System in Radiooncology. The history of the use of neutrons in radiotherapy will be revisited by focusing on the ideas, theories and experiments that led to first clinical studies. For addressing epistemological questions regarding biological effects of fast neutrons, the notion of an "experimental system" is employed and its evolution over time discussed. Taking up the analytical framework of Hans-Jörg Rheinberger, biological effects and the physical instrument "cyclotron" are conceptualized in terms of epistemic and technical objects. It is shown that the epistemic object "biological effects of neutrons" could never reach the status of a technical object and - more strikingly - that the cyclotron itself returned to being an epistemic object in the context of medicine. So there is a misunderstanding of the different roles epistemic and technical objects have to play in the framework of an experimental system. This misunderstanding led to today's highly controversial discussions about the application of neutrons in radiotherapy.