1. A method, implemented by a processor, for performing auto-contouring in a medical image, the method comprising: receiving the medical image, wherein the medical image contains a region of interest and a plurality of voxels; identifying a first contour, by the processor, using a boundary detector, wherein the first contour is formed by a first set of voxels of the medical image located on a surface of the region of interest; refining the first contour, by the processor, based on a shape dictionary, to generate a second contour of the region of interest; and updating the boundary detector, by the processor, based on a second set of voxels located along the second contour.