Mammographic Abnormalities

As Kopans [94] describes in his work, there is a large number of types of abnormalities that can be present in a breast. Among those, signs of breast cancer are normally associated with:

• Asymmetry between images of left and right breasts.
• Distortion of the normal architecture of the breast tissue.
• Presence of micro-calcifications in the breast.
• Presence of masses in the breast.

It is generally accepted that the essential characteristic of a high-quality mammogram is the ability to visualize these four features [4,164,190].

Both breasts are usually considered as almost symmetric structures. While exact mirror images are not to be expected when comparing them (usually the first practice of expert physicians), the tissue patterns within each breast should be similarly distributed. An asymmetric area may be indicative of a developing mass or a variation of normal breast tissue.

A distortion in the normal breast architecture (architectural distortion) refers to a derangement or disruption of the normal arrangement of the tissue strands of the breast resulting in a radiating or haphazard pattern without an associated visible centre. This includes spiculations radiating from a point, and focal retraction or distortion of the edge of the parenchyma.

Micro-calcifications are tiny calcifications that range from $50$ to several hundred microns in diameter, which usually appear in clusters. In these cases, they are analyzed according to their size, shape, number, and distribution. The general rule is that larger, round or oval shaped calcifications with uniform size have a higher probability of being associated with a benign process, whereas smaller, irregular, polymorphic, branching calcifications heterogeneous in size and morphology are more often associated with a malignant process.

Figure 1.2: The shape and margin of a mass are strong signs of their malignancy/benignancy degree. Image extracted from the web of GE Healthcare [57].

Mass Shapes		Mass Margins

A breast mass, on the other hand, is a localized swelling, protuberance, or lump in the breast, which usually is described by its location, size, shape, margin characteristics, and any other associated findings (i.e. architectural distortion, X-ray attenuation). Depending on a morphologic criteria, the likelihood of malignancy can be established. Figure  shows schematically different masses according to their shape and margin. Normally, a benign process is associated with the presence of circular or oval shapes, while, in contrast, spiculated masses are more probable to be the sign of a malign process. The margin refers to the border of a mass, and it should be examined carefully because it is one of the most important criteria in determining whether the mass is the result of a benign or malign process. Radiologists classify the margin among five classes [72]:

• Circumscribed margins, which are well defined and sharply demarcated with an abrupt transition between the lesion and the surrounding tissue.
• Obscured margins, which are hidden by superimposed or adjacent normal tissue.
• Micro-lobulated margins, which have small undulating circles along the edge of the mass.
• Ill-defined margins, which are poorly defined and scattered.
• Spiculated margins, which are marked by radiating thin lines.

The probability to find a malignancy mass is normally ordered according to this classification. The more ill-defined and spiculated the margin, the higher the probability to be associated with a malignant process. It should be clear that these morphological aspects can be very subtle and difficult to diagnose, even for an expert radiologist. Figure  shows three different regions of interest, including expert annotations.

Figure 1.3: Three mass examples with different shape and margin. The last one has an increased probability to be malignant.

Circular Shape	Lobular Shape	Spiculated Shape
Circumscribed Margin	Well Defined Margin	Ill Defined Margin