A gantry structure (16) designed for pivoting about an axis of rotation (18) and for delivering a hadron beam on a target comprises a beam delivery line (24) receiving the hadron beam in a direction essentially parallel to the axis of rotation (18), deviating it away from said axis of rotation (18) and delivering it so that the axis of the beam intersects the axis of rotation (18). The beam delivery line (24) is a self-supporting structure supported by a pivotable support arm (32) in proximity of the center of gravity of the beam delivery line (24). A counterweight (42) is supported by a support arm extension (40) on the other side of the axis of rotation (18) for compensating the moment of force due to the weight of the beam delivery line (24) and the support arm (32).
