Methods, systems, and non-transitory machine-readable storage medium for volumetric segmentation of structures in planar medical images. The method includes, for example, receiving a plurality of planar medical images including a structure and displaying a first planar medical image. The method also includes determining and displaying a first two dimensional (2D) contour of the structure using a first single straight line segment between user-selected first and third locations in the first planar medical image. The method further includes determining and displaying a second 2D contour of the structure using a second single straight line segment between user-selected first and second locations in the first planar medical image. The method also includes determining a three dimensional (3D) contour of the structure using the second single straight line segment. The method further includes determining a long axis of the structure using the 3D contour and outputting a dimension of the long axis.