What Mammographic Asymmetry Means
A mammographic asymmetry refers to an area of breast tissue that appears different when comparing one breast to the other, or when examining the same breast from different angles on a mammogram. This difference indicates that the tissue in one area is denser or more prominent than expected, without forming a distinct mass with defined borders.
There are several ways an asymmetry can present on a mammogram. A “global asymmetry” describes an overall difference in density between the two breasts. “Focal asymmetry” refers to a localized area of increased density that is not clearly a mass and is seen in only one projection, while “developing asymmetry” is a new or increasing area of asymmetry compared to previous mammograms.
Common Non-Cancerous Causes of Asymmetry
Many factors can cause mammographic asymmetry, and most are not related to cancer. Normal variations in breast tissue density are common, as breast tissue naturally varies in distribution and thickness among individuals. Hormonal changes throughout a woman’s life, such as during menstruation, pregnancy, breastfeeding, or with the use of hormone therapy, can also influence breast tissue appearance, leading to temporary or persistent asymmetries.
Benign conditions frequently present as asymmetries on mammograms. Fluid-filled sacs, known as cysts, can create areas of increased density. Inflammation or infections within the breast tissue can also lead to localized swelling and density changes that appear as asymmetry. Additionally, scar tissue resulting from prior breast surgeries, biopsies, or even past injuries can mimic suspicious findings by creating areas of architectural distortion or increased density.
An apparent asymmetry can also be due to the superimposition of normal breast tissue. Overlapping healthy structures on 2D mammogram images can create the illusion of a denser area, which additional imaging helps differentiate from true tissue changes.
When Asymmetry May Indicate Concern
While most asymmetries are benign, certain characteristics can suggest a need for closer examination. An asymmetry that is new on a current mammogram and was not visible on previous studies, or one that has clearly increased in size or density over time, raises a higher level of concern.
An asymmetry becomes more suspicious if it is found in conjunction with other concerning features. These features include the presence of fine, suspicious calcifications within the asymmetric area, which are tiny calcium deposits that can sometimes be associated with early breast cancer. Similarly, if the asymmetry is accompanied by architectural distortion, where the normal tissue pattern appears pulled or distorted without a clear mass, it can also indicate a potential issue.
The presence of a palpable lump in the area of the mammographic asymmetry further increases the level of suspicion. When a physical finding correlates with an imaging finding, it prompts more immediate evaluation, suggesting the asymmetry may represent a developing abnormality.
Next Steps After an Asymmetry Finding
Following the detection of an asymmetry on a screening mammogram, additional imaging is typically recommended. This often begins with specialized mammogram views, such as spot compression or magnification views, which apply localized pressure to better visualize the area. These views help determine if the asymmetry is merely overlapping normal tissue or a true area of concern.
Breast ultrasound is another common next step, providing real-time images that can differentiate between solid masses, fluid-filled cysts, or areas of normal glandular tissue. Ultrasound is particularly useful for evaluating focal asymmetries, as it can often confirm if a finding is benign or needs further attention. For some asymmetries, especially those with subtle or complex features, a breast MRI may be recommended to provide more detailed images of the breast tissue.
If these additional imaging studies still raise concern, a biopsy may be performed to obtain a tissue sample for microscopic examination, providing a definitive diagnosis. However, it is important to remember that many asymmetries resolve as benign findings after further imaging, and a biopsy is only necessary in a smaller proportion of cases.