The present invention refers to a radiation system (1) comprising an excentric gantry (100) arranged in connection with multiple treatment rooms (61-68) separated by radiation-shielding separating members (71-78). A movable rotation head (120) is connected to the gantry (100) and is able to move between, and direct a radiation beam (110) into, the treatment rooms (61-68). A simulator head (200-1 to 200-8) is preferably arranged together with the radiation system so it can be used in each respective treatment room (61-68). In such a case, while a first subject (40-1) is being irradiated in a first room (61), a treatment set-up procedure, including correct positioning of subjects (40-2 to 40-8) and irradiation simulation, can simultaneously take place for the other subjects (40-2 to 40-8) in the other treatment rooms (62 to 68).