A device is provided for positioning a target volume, such as a phantom or a patient, in a radiation therapy apparatus. The apparatus directs a radiation beam towards the target. The apparatus can include a target support whereon the target is immobilized, a two dimensional radiation detector fixed with fixations means in a known geometric relationship to the target support, the radiation detector being capable of detecting the position of intersection of the radiation beam with the detector, and correcting means for correcting the relative position of the beam and the target support, based on the detected intersection position.