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, by the processor, voxels of the medical image which are on a surface of the region of interest; selecting, by the processor, a first set of the identified voxels to form a first contour; refining the first contour, by the processor, based on a shape dictionary, to generate a second contour of the region of interest; updating a boundary detector based on a seconds set of the identified voxels, the second set being located along the second contour; and applying the updated boundary detector to auto-contour the region of interest in the medical image.