As an ongoing effort to develop a computer aided system for the detection of masses on mammograms, we propose on this work a new model-based segmentation algorithm. The algorithm is based on a template matching scheme by using the mutual information approach in the similarity metric. Thus, the system will be able to determine if it exists a true mass on the studying image. The proposal was developed and evaluated using a database of 120 mammograms, 40 mammograms with confirmed masses and 80 normal ones. CAD performance was assessed using Receiver Operating Characteristics (ROC) and Free Receiver Operating Characteristics (FROC) analysis. The results prove the validity of the proposed method.