1. 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 radially and vertically relative to the horizontal longitudinal axis ( 16 ) deflect a particle beam ( 12 ) that has been axially injected by a particle beam accelerator, with an extensive rotationally-symmetrical primary structure ( 18 ) and a secondary structure ( 30 ) supported by the primary structure ( 18 ) and holding the magnets ( 36, 38, 40 ), with the secondary structure ( 30 ) having a rigidity that is designed such that vertical displacements of the magnets ( 36, 38, 40 ) due to their weight are essentially of the same magnitude (isokinetic) in all the angle of rotation positions of the gantry arrangement ( 10 ).