A mammogram is an X-ray image of the breast, a primary tool used for breast cancer screening and early detection. It helps identify changes in breast tissue that might be too small to feel. One common finding on mammograms that can cause concern is “asymmetry,” which indicates a difference in breast tissue appearance. While an asymmetry finding often leads to further investigation, it is frequently not indicative of cancer.
What Mammogram Asymmetry Means
Mammogram asymmetry refers to a difference in the density or appearance of breast tissue between the two breasts, or within different parts of the same breast, that does not fit the typical pattern of normal breast tissue. For instance, it might appear as an area of increased density in one breast compared to the same location in the opposite breast.
Radiologists categorize asymmetry into various types based on its presentation. A “focal asymmetry” is a localized area of increased density visible on two different mammogram views, which is smaller than a full breast quadrant and lacks the convex-outward borders typical of a mass. “One-view asymmetry” is seen on only a single mammogram projection. “Global asymmetry” involves a larger area of increased tissue density, encompassing more than one quadrant of the breast. A “developing asymmetry” is a focal asymmetry that is new, larger, or more noticeable compared to previous mammograms, which can be a more concerning finding.
The Likelihood of Asymmetry Being Cancer
The detection of asymmetry on a mammogram often causes anxiety, but the vast majority of these findings are not cancerous. About 3% of mammograms show breast asymmetry, and most are due to overlapping breast tissues rather than a serious condition.
While most asymmetries are benign, the likelihood of cancer varies depending on the type and associated features. For instance, focal asymmetry is seen in about 21% of all mammograms and has a low chance of malignancy, often less than 2%. However, a “developing asymmetry,” which is new or has increased in size or density since prior exams, carries a higher risk, with about a 12-15% chance of being cancerous. Factors that might increase suspicion include associated calcifications or architectural distortion, though even with these, the finding often remains benign upon further investigation.
Next Steps After an Asymmetry Finding
If an asymmetry is identified on a screening mammogram, additional imaging is recommended to get a clearer picture of the area. This callback is a standard protocol for thorough evaluation and does not necessarily indicate cancer. About 1 in 10 mammograms result in a callback, and in approximately 90% of these cases, further testing reveals nothing serious.
The next steps often involve a diagnostic mammogram, which includes additional views such as spot compression and magnification views, to better characterize the asymmetry. Digital breast tomosynthesis (3D mammogram) may also be used, as it provides a more detailed view by slicing through breast tissue layers, helping distinguish true asymmetry from overlapping tissues. A targeted ultrasound of the area of concern is almost always performed to determine if the asymmetry represents a solid mass, fluid-filled cyst, or normal overlapping breast tissue. If these additional imaging techniques still show suspicious features or do not resolve the finding, a breast biopsy might be recommended to obtain tissue for microscopic examination and confirm whether the cells are benign or malignant.
Common Benign Causes of Asymmetry
Asymmetry on a mammogram is frequently due to non-cancerous factors. Normal variations in breast tissue density are a common cause, as breasts naturally differ slightly in size, shape, and density. Hormonal changes throughout a woman’s life, such as those occurring during puberty, menstrual cycles, pregnancy, and menopause, can also lead to temporary or persistent asymmetries. For example, hormone replacement therapy can increase breast density, which may contribute to asymmetry.
Benign breast conditions can also manifest as asymmetry. These include fibrocystic changes, which involve the development of fluid-filled cysts and scar-like tissue, making areas of the breast feel denser or lumpy. Other non-cancerous causes include benign growths like fibroadenomas, dense stromal fibrosis where connective tissue is denser, or even simple positioning differences during the mammogram itself. Scarring from previous surgeries or biopsies can also appear as an area of asymmetry on subsequent mammograms.