A mammogram result containing the term “focal asymmetry” (FA) often generates anxiety. This finding represents an area of unequal tissue density observed in one breast compared to the corresponding region in the other. While asymmetry suggests a variation, it is a common finding that requires clarification, not an immediate diagnosis of cancer. Understanding FA and the steps taken to investigate it provides clarity regarding its significance. The evaluation process is designed to efficiently distinguish between benign causes and the small percentage that may indicate malignancy.
What is Focal Asymmetry?
Focal asymmetry is a specific description used by radiologists for a localized area of dense fibroglandular tissue seen on two different mammographic views. This distinguishes it from a simple asymmetry (visible on only one projection) or a global asymmetry (involving a large portion of the breast). An FA does not have the classic, space-occupying shape of a true mass, which usually presents with convex borders. Instead, the borders of an FA are typically ill-defined or interspersed with fatty tissue.
The appearance of an FA often results from the superimposition of normal, overlapping breast structures, creating a denser shadow on the X-ray image. This localized patch of increased density can be a real lesion or an illusion caused by how tissue structures line up during mammogram compression. Because FA is a unilateral finding, present in one breast but not the other, it requires additional investigation to rule out an underlying abnormality.
The Standard Diagnostic Workup
When a focal asymmetry is identified on a routine screening mammogram, the patient is recalled for a diagnostic workup. The first step involves specialized, additional mammographic views, such as spot compression and magnification views. Spot compression applies targeted pressure to the area, often causing the density to spread out and dissipate. This confirms the finding was merely an artifact of overlapping normal tissue, known as a summation artifact.
If the density persists or becomes more apparent, the next procedure is a targeted ultrasound of the area. Ultrasound determines whether the density corresponds to a solid mass, a fluid-filled cyst, or if no abnormality is visible, which often occurs when the area is simply dense fibroglandular tissue. If a solid lesion is identified, the radiologist assesses its shape and margins for suspicious features.
The workup guides the final assessment, assigned using the Breast Imaging Reporting and Data System (BI-RADS). An FA found to have no suspicious characteristics, and which either disappears or is confirmed as solid but likely benign, is often classified as BI-RADS Category 3. This category means the finding is “probably benign” and recommends a short-interval follow-up, typically in six months, instead of an immediate biopsy. If suspicious features are present or a developing asymmetry is identified, the finding may be upgraded to BI-RADS Category 4, indicating a suspicion of malignancy that requires a tissue biopsy.
Statistical Risk of Malignancy
The likelihood of a focal asymmetry representing cancer is low, which is reassuring for most patients. The majority of FAs detected on screening mammograms are benign findings, often resolving upon further imaging with spot compression and ultrasound. When a solitary FA is classified as BI-RADS 3—meaning it persists after the initial workup but lacks other suspicious features—the probability of malignancy is very low, typically 2% or less.
The most common causes for FA are normal anatomical variations, such as localized areas of dense fibroglandular tissue or a summation of overlapping normal structures. Other benign causes include post-surgical changes, scars, or conditions like cysts and sclerosing adenosis. If an FA is persistent and requires a biopsy, the malignancy rate can increase, but the overall chance of cancer remains low for this specific finding.
Malignant causes, though uncommon, include invasive ductal carcinoma and invasive lobular carcinoma, which often presents subtly as an asymmetry rather than a distinct mass. The probability of malignancy increases if the asymmetry is classified as a “developing asymmetry,” meaning it is new, larger, or denser compared to prior mammograms. A developing asymmetry may carry a malignancy rate ranging from 10% to 15%, leading to a higher level of suspicion and often a direct biopsy recommendation.
Visual Characteristics That Increase Suspicion
While a simple focal asymmetry is usually a low-suspicion finding, specific visual characteristics can elevate the concern for malignancy. Radiologists examine the asymmetry for associated findings that indicate an underlying cancerous process. These features include:
- Architectural distortion, where breast tissue appears pulled or tethered without a clearly defined mass.
- The presence of suspicious microcalcifications, which may indicate ductal carcinoma in situ.
- Irregular or spiculated borders.
- Any change in the finding over time, known as an interval change.
A developing asymmetry is treated with urgency due to its higher malignancy risk.