The breasts are seen as symmetric structures and should be compared as such. While exact mirror images are not to be expected, from an overall vantage point the tissue patterns within each breast should be similarly distributed. An asymmetric area may be indicative of a developing mass, a variation of normal breast tissue, postoperative change from a previous biopsy, or merely poor positioning and compression during imaging. The appearance of asymmetries due to positioning and compression during imaging is often the result of superimposition of normal breast structures. True breast asymmetry, on the other hand, is three-dimensional and should be present on both MLO and CC views. Once an asymmetry is determined to be three-dimensionally real, the interpreter must determine whether the asymmetry is a benign variation of asymmetric breast tissue or a focal asymmetric density that may represent a significant mass. If the former determination could not be made, further evalution is necessary to clarify if the focal asymmetric density may possibly be a malignant process.

The central difference between a focal asymmetric density and normal asymmetric breast tissue is the appearance of the former's density to be concentrated toward its center and to be forming a mass with a dense central zone. Confirmation is made by its recognition on more than two views, and the density tends to be concentrated rather than diffuse. Closer inspection, spot compression, and magnification may confirm that the focal asymmetric density is a mass. Benign asymmetric tissue, on the other hand, tends to be larger and more diffuse with a morphology and pattern similar to breast tissue in the contralateral breast. Closer inspection will reveal the absence of a radiodense center, diffused margins, and no associated mass formation or calcifications. Another helpful guideline is that normal breast parenchyma often has fat traversing it. Breast cancer usually does not contain fat. Finally, normal asymmetric density must not be a neodensity as it should not develop between successive mammograms.

In conclusion, asymmetric density is often the first impression that the interpreter notices. It requires closer scrutiny and possibly further evaluations because asymmetric density may be a localizing sign of breast cancer. The management of a case, i.e. the decision to follow-up or to biopsy, will depend upon the interpretation of the asymmetric density. Note, however, that mammography interpretation is never done in a vacuum, and in addition to the mammographic findings, the history and physical examination of the patient play equally important roles.