The invention is related to 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, by which a given geometrical figure can be projected onto the three-dimensional surface of the patient's body, at least one control device and at least one optical sensor device, by which the geometrical figure projected onto the surface of the patient's body can be acquired and the acquired data can be supplied to the control device, wherein the control device is realized to determine the three-dimensional coordinates of the geometrical figure projected onto the patient's body from the data acquired by the sensor device, and wherein the control device is realized to compare the determined three-dimensional coordinates of the geometrical figure projected onto the patient's body with desired three-dimensional coordinates.