Mass boundary segmentation plays an important role in computer aided diagnosis (CAD) system. Since the shape and boundary are crucial discriminant features in CAD, the active contour methods are more competitive in mass segmentation. However, the general active contour methods are not so effective for some cases, because most masses possess very blurry margin that easily induce the contour leaking. To the end, this paper presents an improved geometric active contour for mass segmentation. It firstly introduces the morphological concentric layer model for automatically initializing. Then an embedded level set is used to extract the adaptive shape constraints. For refining the boundary, a new shape constraint function and stopping function are designed for the enhanced geometric active contour method. The proposed method is tested on real mammograms containing masses, and the results suggest that the proposed method could effectively restrain the contour leaking and get better segmented results than general active contour methods.