An image processing system for contouring three-dimensional medical image data, the system comprising: a processing unit and first, second and third interfaces (110, 120, 130), the first interface being configured to enable the processing unit access to source data in the form of the three-dimensional medical image data, which source data represent an anatomic structure of interest that is to be contoured, and tissues adjoining the anatomic structure of interest; the processing unit being configured to select a first subset of the source data defining a first two-dimensional graphic representation of the source data, the first subset being arranged in a first image slice through the anatomic structure of interest at a first axial position in the source data; the second interface being configured to, in response to control instructions from the processing unit, output the first subset of the source data for presentation on a graphical display; and the third interface being configured to forward a first set of instructions ( edge) to the processing unit, the first set of instructions ( edge) being received in the third interface in the form of user commands (ucmd1, ucmd2) and identifying a first edge of the anatomic structure of interest in the first image slice, after having received the first set of instructions ( edge), the processing unit is configured to: select a second subset of the source data defining a second two-dimensional graphic representation of the source data, the second subset being arranged in a second image slice through the anatomic structure of interest at a second axial position in the source data, which second axial position is different from the first axial position; generate control instructions configured to cause the second interface to output the second subset of the source data for presentation on the graphical display; receive, via the third interface, a second set of instructions ( edge), the second set of instructions being received in the form of user commands (ucmd1, ucmd2) and identifying a second edge of the anatomic structure of interest in the second image slice; calculate a three-dimensional shell based on the first and second edges (E1; E2) and the source data, the three-dimensional shell representing an approximation of a delimiting surface of the anatomic structure of interest, wherein the calculation comprises to calculate an initial estimate of the three-dimensional shell between the first and second axial positions (z1; z2) in the source data by interpolating between first and second sets of data points, the first set of data points describing a first outline of the first edge and the second set of data points describing a second outline of the second edge; analyze a first data portion of the first subset of the source data in proximity to the first edge; analyze a second data portion of the second subset of the source data in proximity to the second edge, and based thereon derive a set of image features defining an estimated perimeter of the anatomic structure of interest, and calculate a subsequent estimate of the three-dimensional shell on the further basis of the set of image features, characterized in that the processing unit is further configured to interpolate the set of image features defining the estimated perimeter over the initial estimate by using harmonic functions, and where image features derived from the first and second data portions (B1; B2) represent boundary conditions for the interpolation process. select a second subset of the source data defining a second two-dimensional graphic representation of the source data, the second subset being arranged in a second image slice through the anatomic structure of interest at a second axial position in the source data, which second axial position is different from the first axial position; generate control instructions configured to cause the second interface to output the second subset of the source data for presentation on the graphical display; receive, via the third interface, a second set of instructions ( edge), the second set of instructions being received in the form of user commands (ucmd1, ucmd2) and identifying a second edge of the anatomic structure of interest in the second image slice; calculate a three-dimensional shell based on the first and second edges (E1; E2) and the source data, the three-dimensional shell representing an approximation of a delimiting surface of the anatomic structure of interest, wherein the calculation comprises to calculate an initial estimate of the three-dimensional shell between the first and second axial positions (z1; z2) in the source data by interpolating between first and second sets of data points, the first set of data points describing a first outline of the first edge and the second set of data points describing a second outline of the second edge; analyze a first data portion of the first subset of the source data in proximity to the first edge; analyze a second data portion of the second subset of the source data in proximity to the second edge, and based thereon derive a set of image features defining an estimated perimeter of the anatomic structure of interest, and calculate a subsequent estimate of the three-dimensional shell on the further basis of the set of image features,