Device for positioning a target volume (112) such, as a phantom or a patient in a radiation therapy apparatus, said apparatus directing a radiation beam (405) towards said target (112), characterized in that it comprises:—a target support (100) whereon the target is immobilized; a two dimensional radiation detector (103) fixed with fixations means (101, 102, 104, 106 107; 301, 302, 304, 305, 306; 208, 209) in a known geometric relationship to said target support (100), said radiation detector (103) being capable of detecting the position of intersection of said radiation beam (105) with said detector (103); correcting means for correcting the relative position of said beam (105) and said target support 100), based on said detected intersection position.