Focal and Developing Asymmetries

A focal asymmetry is a discrete area of fibroglandular tissue density visible on two mammographic views, occupies less than one quadrant of the breast, and differs from a mass in that it lacks a discernible margin. A focal asymmetry may or may not contain fat. When encountered in screening mammography, it frequently warrants further evaluation with spot compression views and ultrasonography. When non-palpable nor associated with additional imaging findings such as calcifications, architectural distortion, or sonographic abnormalities, a focal asymmetry encountered on a baseline mammogram has a likelihood of malignancy of less than 1%. Most frequently, a focal asymmetry represents an island of normal breast tissue, with only 3% of breast cancers presenting as focal asymmetries. It is important not to confuse focal asymmetries with developing asymmetries, with the former representing baseline mammographic findings or stable findings established by multiple prior comparison mammograms and the latter representing a new, larger, or denser focal asymmetry compared to prior mammograms. The likelihood of a developing asymmetry representing a malignancy may range from 13% to 27%. Common benign causes of focal and developing asymmetries represent benign breast tissue, fibrocystic and fibroadenomatoid changes, PASH, fat necrosis, mastitis, fibrosis, gynecomastia (in men), and posttreatment changes. Malignant causes are less common but include the various types of breast cancer and lymphoma.