1. An apparatus for changing a contour of a lesion in a medical image, the apparatus comprising: a memory configured to store instructions; and at least one processor, upon executing the stored instructions, configured to: generate a first contour of a lesion in a first image frame which is at least one of two dimensional image frames that form at least one three-dimensional image, generate a reference lesion contour, in a target image frame which is different from the first image frame, based on at least one portion of information about pixels in the target image frame, modify the reference lesion contour to produce a plurality of lesion contour candidates, and conform at least one of the lesion contour candidates as a second contour of a lesion in the target image frame.