1. A device for positioning a target in a patient in a radiation therapy apparatus, the apparatus directing a radiation beam towards the target the device comprising a target support for immobilizing the target; a two-dimensional radiation detector fixed with fixation device in a predetermined geometric relationship to the target support the radiation detector configured to detect an intersection position of the radiation beam with the detector; and correcting device configured to correct a relative position of the beam and the target support, based on the detected intersection position.