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 arranged in a first image slice through the anatomic structure of interest, the first image slice having a first orientation 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 form of user commands (ucmd1) and identifying a first edge of the anatomic structure of interest in the first image slice, and 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 arranged in a second image slice through the anatomic structure of interest, the second image slice having a second orientation in the source data, which second orientation is different from the first orientation; 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 ( edge) being received in the form of user commands (ucmd2) and identifying a second edge of the anatomic structure of interest in the second image slice; and 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 is configured to generate a first iteration of a triangular mesh structure approximating the three-dimensional shell to an estimated perimeter of the anatomic structure of interest, the triangular mesh structure including a set of vertices (v) each in which a number of triangles of the mesh structure meet, characterized by the first iteration of the triangular mesh structure involving an analysis of: a first intersection between the three-dimensional shell and a first plane defined by the first image slice, and a second intersection between the three-dimensional shell and a second plane defined by the second image slice, the analysis comprising, for a triangle (Tn) of the triangular mesh structure forming a part of the first intersection: searching for an image point on the first edge along a direction of a projected surface normal (n) to said triangle (Tn), the image point being located within a search range from the intersection, and if such an image point is found assigning an image feature of the image point found to the triangle (Tn) of the triangular mesh structure; and for a triangle of the triangular mesh structure forming a part of the second intersection: searching for an image point on the first edge along a direction of a projected surface normal to said triangle, the image point being located within a search range from the intersection, and if such an image point is found assigning an image feature of the image point found to the triangle of the triangular mesh structure. a first intersection between the three-dimensional shell and a first plane defined by the first image slice, and a second intersection between the three-dimensional shell and a second plane defined by the second image slice, the analysis comprising, searching for an image point on the first edge along a direction of a projected surface normal (n) to said triangle (Tn), the image point being located within a search range from the intersection, and if such an image point is found assigning an image feature of the image point found to the triangle (Tn) of the triangular mesh structure; and searching for an image point on the first edge along a direction of a projected surface normal to said triangle, the image point being located within a search range from the intersection, and if such an image point is found assigning an image feature of the image point found to the triangle of the triangular mesh structure.