Mammography: Asymmetries, Masses, and Architectural Distortion

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Right- and left-breast mammograms are traditionally displayed back-to-back, projection for projection, to facilitate the perception of areas of asymmetry, which may on occasion be the only manifestation of breast cancer on standard mammographic views. Asymmetry is usually distinguished from mass in that it demonstrates concave-outward rather than the convex-outward contours typical of a mass, and that it is interspersed with fat rather than appearing denser in the center than at the periphery (typical of a mass). There are four types of asymmetry, all of which represent areas of fibroglandular-density tissue that is more extensive in one breast than the other: 1. Summation artifact: This is caused by superimposition of normal fibroglandular breast structures on a given mammographic projection. As such, this finding will be visible on only one of the two standard views and will not be seen on additional views taken in different projections. One must distinguish summation artifact from the other types of asymmetry described below, because it never represents breast cancer and should not be subjected to biopsy. A one-view-only mammographic finding is called an asymmetry prior to diagnostic imaging evaluation. Most (>75%) asymmetries represent summation artifacts; the rest are found to be other types of asymmetry or masses. 2. Global asymmetry: formerly called asymmetric breast tissue, represents a greater volume of fibroglandular tissue in one breast than in the corresponding location in the opposite breast, without associated mass, microcalcification, or architectural distortion. It is found in approximately 3% of screening mammography examinations. It almost always represents a normal variant, but occasionally it may indicate the presence of an underlying breast cancer if it corresponds to a palpable abnormality. In this latter clinical setting, the radiologist usually will recall the patient for additional imaging evaluation. 3. Focal asymmetry: formerly called focal asymmetric density, is an asymmetry of fibroglandular-tissue density seen on two different mammographic projections but lacking the convex-outward contours and conspicuity of a mass. It usually represents an island of normal dense breast tissue, but its lack of specific benign characteristics may warrant further evaluation, especially if it is not interspersed with fat. In a retrospective study of 300 consecutive nonpalpable breast cancers, 3% were identified as focal asymmetries. The likelihood of malignancy among findings characterized as focal asymmetry (without associated mass, calcifications, architectural distortion, sonographic abnormality, or palpable correlate) is <1%. 4. Developing asymmetry: formerly called developing density or neodensity, is a focal asymmetry that is new, larger, or denser on current examination than previously. To identify such a lesion, one must have prior mammograms available for comparison. In a retrospective study of 300 consecutive nonpalpable breast cancers, 6% were identified as developing asymmetries. The likelihood of malignancy among findings characterized as developing asymmetry is approximately 15%.