A method for volumetric segmentation of an anatomical structure in a plurality of planar medical images, the method comprising: receiving, at an electronic processor, the plurality of planar medical images, wherein the plurality of planar medical images form a three dimensional, 3D, volume including the structure; displaying, on a display, a first planar medical image from the plurality of planar medical images; receiving, with a user interface, a user input indicating a line segment in the first planar medical image, wherein the line segment provides an approximate diameter of a region of interest; determining, with the electronic processor, an inclusion region of the 3D volume using the line segment, wherein the inclusion region consists of a portion of the structure; determining, with the electronic processor, a containment region of the 3D volume using the line segment, wherein the containment region includes the structure; determining, with the electronic processor, a background region of the 3D volume using the line segment, wherein the background region excludes the structure; determining, with the electronic processor, a 3D contour of the structure using conditional densities from a function of the histogram of a first set of voxels located within the inclusion region, the containment region, and conditional densities from a function of the histogram of a second set of voxels located within the background region, and prior probabilities of classifying a voxel as being a member of a foreground class or of a background class, the prior probabilities varying as a function of a distance from the inclusion region and the containment region; determining, with the electronic processor, a long axis of the structure using the 3D contour of the structure; and outputting, with the electronic processor, a dimension of the long axis of the structure. receiving, at an electronic processor, the plurality of planar medical images, wherein the plurality of planar medical images form a three dimensional, 3D, volume including the structure; displaying, on a display, a first planar medical image from the plurality of planar medical images; receiving, with a user interface, a user input indicating a line segment in the first planar medical image, wherein the line segment provides an approximate diameter of a region of interest; determining, with the electronic processor, an inclusion region of the 3D volume using the line segment, wherein the inclusion region consists of a portion of the structure; determining, with the electronic processor, a containment region of the 3D volume using the line segment, wherein the containment region includes the structure; determining, with the electronic processor, a background region of the 3D volume using the line segment, wherein the background region excludes the structure; determining, with the electronic processor, a 3D contour of the structure using conditional densities from a function of the histogram of a first set of voxels located within the inclusion region, the containment region, and conditional densities from a function of the histogram of a second set of voxels located within the background region, and prior probabilities of classifying a voxel as being a member of a foreground class or of a background class, the prior probabilities varying as a function of a distance from the inclusion region and the containment region; conditional densities from a function of the histogram of a first set of voxels located within the inclusion region, the containment region, and conditional densities from a function of the histogram of a second set of voxels located within the background region, and prior probabilities of classifying a voxel as being a member of a foreground class or of a background class, the prior probabilities varying as a function of a distance from the inclusion region and the containment region; determining, with the electronic processor, a long axis of the structure using the 3D contour of the structure; and outputting, with the electronic processor, a dimension of the long axis of the structure.