A mammogram serves as a routine screening tool, using low-dose X-rays to look for changes in breast tissue that could indicate cancer. During this process, a radiologist compares the images of the left and right breasts, looking for symmetry. When the tissue density in a corresponding area of one breast appears different from the other, the finding is termed “asymmetry.”
This report of asymmetry can cause significant anxiety, but it is a frequent finding on screening mammograms and is most often benign. The term simply signals an unequal distribution of fibroglandular tissue, which requires a closer look to determine its nature. It is an alert that prompts further investigation, not a diagnosis of cancer.
Defining Mammographic Asymmetry
Mammographic asymmetry refers to an area of increased fibroglandular density in one breast that does not have the clearly defined, convex borders of a true mass. This finding is a difference in tissue structure between the two breasts, seen when comparing the same projection, such as the craniocaudal (CC) or mediolateral oblique (MLO) view. The most common reason for an asymmetry is the normal superimposition of healthy breast structures, often called a summation artifact, which can mimic a true lesion on a two-dimensional image.
Unlike a mass, which is a three-dimensional, space-occupying lesion with distinct margins, an asymmetry is generally interspersed with fat and lacks a dense center. This unequal appearance is often related to natural variations in how glandular tissue is distributed in each breast. Asymmetries are common, appearing in approximately 3% of all mammograms.
Classifying Asymmetry Findings
Radiologists use a standardized lexicon, part of the Breast Imaging-Reporting and Data System (BI-RADS), to classify these findings. The simplest finding is an “asymmetry,” which is a dense area visible only on one mammographic projection. A “focal asymmetry” is more localized and is visible on two different mammographic views, such as both the CC and MLO projections, but still lacks the definitive shape of a mass.
A “global asymmetry” is a large area of dense tissue that is visible on both projections and occupies a greater volume, typically spanning more than one quadrant of the breast. This type is often considered a normal variant and is the least concerning, frequently representing only a natural difference in breast development or hormonal influence. The most concerning category is the “developing asymmetry,” which is a focal asymmetry that is either new or has become noticeably larger or more conspicuous since the patient’s last mammogram.
Because breast tissue typically becomes less dense with age, a developing asymmetry signals a change that warrants immediate diagnostic workup. A finding of asymmetry on a screening mammogram usually results in a BI-RADS 0 assessment, which means the evaluation is incomplete and requires additional imaging to clarify the finding.
The Follow-Up Diagnostic Process
Once an asymmetry is identified on a screening mammogram, the patient is recalled for a diagnostic workup to move from the BI-RADS 0 category to a final, definitive classification. This process begins with a diagnostic mammogram, which includes specialized views to better characterize the area of concern. Spot compression views are a primary tool, applying localized pressure to the specific area of asymmetry to spread out the overlapping tissue.
If the asymmetry disappears or resolves with this increased compression, it confirms the finding was merely a summation of normal, overlapping breast tissue. If the asymmetry persists on the spot compression views, the next step is typically a targeted ultrasound of the area. Ultrasound uses sound waves to determine if the dense area is solid tissue, a simple fluid-filled cyst, or just dense glandular tissue.
If the asymmetry is confirmed as a solid mass, or if it persists without a clear benign explanation, further action is necessary. These steps are designed to rule out malignancy and, in the vast majority of cases, the asymmetry is resolved as a normal, non-cancerous finding.
Interpreting the Final Results
After the diagnostic workup is complete, the radiologist assigns a final BI-RADS category to the finding. Most asymmetries are ultimately determined to be normal anatomical variants, hormonal changes, or scar tissue from a previous surgery, resulting in a BI-RADS 1 (negative) or BI-RADS 2 (benign finding) classification. In these cases, the patient is advised to return for their routine annual screening mammogram.
If the asymmetry remains visible but appears highly likely to be benign, it may be classified as BI-RADS 3, meaning the chance of malignancy is less than 2%. This category requires short-term follow-up, typically with a repeat mammogram in six months, to ensure the finding remains stable over time.
If the asymmetry persists or appears suspicious for malignancy, it will be classified as BI-RADS 4 (suspicious abnormality) or BI-RADS 5 (highly suggestive of malignancy). These higher categories necessitate a tissue biopsy to obtain a definitive diagnosis.