[en] Conceptual and ion-optical designs of an isocentric gantry are presented. The gantry is designed for light-ion therapy using a 6 Tm carbon beam. Its main characteristics are: 5 m overall radius, 1.4 m drift to the patient, 20x10 cm² treatment field, and two-direction active parallel scanning of a fine focused pencil beam. To achieve these features, the beam scanners are optimally positioned upstream the last 90 bending section consisting of two 45 sector magnets with 4 oblique field boundaries and different apertures. The gantry concept is a compromise between 'barrel' and 'conical' gantry shapes and is likely the most space-saving configuration of single-plane isocentric gantries. This new so-called 'short barrel' configuration features also small aperture quadruple lenses, low intermediate dispersion and a fully achromatic beam transport system. Its overall dimensions stay close to the size of the existing proton gantries. (orig.)