1. An apparatus for the representation of a geometrical figure on the surface of a patient's body that is situated on a support, comprising: at least one projection device ( 6 ), by which a given geometrical figure can be projected onto the three-dimensional surface of the patient's body ( 3 ), at least one control device ( 5 ) and at least one optical sensor device ( 20 ), by which the geometrical figure projected onto the surface of the patient's body ( 3 ) can be acquired and the acquired data can be supplied to the control device ( 5 ), wherein the control device ( 5 ) is realised to determine the three-dimensional coordinates of the geometrical figure projected onto the patient's body from the data acquired by the sensor device ( 20 ), and wherein the control device ( 5 ) is realised to compare the determined three-dimensional coordinates of the geometrical figure projected onto the patient's body with desired three-dimensional coordinates.