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 two-dimensional 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; wherein the fixation device comprises fixation frames affixed to the target support, the two-dimensional radiation detector located in close proximity to the target.