Abstract: The invention relates to an isokinetic gantry arrangement for the isocentric guidance of a particle beam (12), that can be rotated about a horizontal longitudinal axis (16) and has a beam optical system, symbolized by magnets (36, 38, 40), that deflects the particle beam (12) axially infected by a particle beam accelerator, radially and vertically relative to the horizontal longitudinal axis (16), with a largely rotationally-symmetrical primary structure (18) and a secondary structure (30) that holds the magnets (36, 38, 40) and is supported by the primary structure (18), with the secondary structure (30) having a rigidity that is designed so that the vertical displacements of the magnets (36, 38, 40) due to their weight are essentially of equal magnitude (isokinetic) in all the angle of rotation positions of the gantry arrangement (10), and a method for its design.