The invention relates to an apparatus for evaluating an activity distribution obtained in a moved target object by a beam that is generated by an irradiation device. Said apparatus comprises: a positron emission tomograph designed to record photons generated in the target object by the beam and generate measurement data representing points of origin of the photons； a movement detection device designed to generate a movement signal representing the movement of the target object； and an evaluation unit designed to associate the points of origin of the measured photons with positions in the target object with the help of the movement signal such that three-dimensional characteristics of the activity distribution actually generated in the target object can be evaluated by means of the photons generated by the beam. The invention further relates to an irradiation system and a method in which such an apparatus is used.
