The invention relates to a medical system for assisting in the planning and/or performance of the reconstruction of normally symmetrical body parts of a patient. The medical system comprises a measuring unit for measuring a first surface and a second surface for determining a topography of the first surface and of the second surface, a storage unit for storing the determined topographies of the first surface and of the second surface, an evaluation unit for establishing a mirror image of the stored topography of the first surface and for calculating deviations of the stored topography of the second surface from the mirror image of the stored topography of the first surface, and a control unit for outputting guidance information for at least one medical instrument on the basis of the calculated deviations of the topography of the second surface from the mirror image of the stored topography of the first surface, in order to reconstruct the topography of the second surface according to the mirror image of the topography of the first surface.