Methods and systems for volumetric segmentation of structures in planar medical images. One example method includes displaying a first planar medical image. The method further includes receiving a user input indicating a line segment in the first planar medical image. The method also includes determining an inclusion region using the line segment. The inclusion region consists of a portion of the structure. The method further includes determining a containment region using the line segment. The containment region includes the structure. The method also includes determining a background region using the line segment. The background region excludes the structure. The method further includes determining a three dimensional (3D) contour of the structure using the inclusion region, the containment region, and the background region. The method also includes determining a long axis of the structure using the 3D contour. The method further includes outputting a dimension of the long axis.
