The term “nodular asymmetry” can be concerning to hear after a screening mammogram, but it is important to understand this terminology is descriptive, not a diagnosis of cancer. Radiologists use this phrase to flag an area where the tissue in one breast appears different from the corresponding area in the other breast. The finding simply indicates that a more detailed look is needed to determine the cause of the variation. This initial finding sets the stage for a comprehensive evaluation, which in the vast majority of cases confirms the asymmetry is due to benign factors.
Defining Nodular Asymmetry
Nodular asymmetry describes a finding where a region of breast tissue appears denser or more prominent in one breast compared to the mirror-image location in the opposite breast. It is a finding of increased fibroglandular density that does not meet the established criteria for a solid mass, lacking the well-defined boundaries of a distinct mass.
This finding is categorized within a broader spectrum of mammographic asymmetries. A simple asymmetry is only visible on one mammographic projection, often representing normal overlapping tissue. Focal asymmetry is seen in two different mammographic views, confirming that the tissue difference is a real three-dimensional finding.
Nodular asymmetry is distinct from global asymmetry, which involves a difference in tissue density across at least one entire quadrant of the breast. It is also different from developing asymmetry, which is a new or increasing focal asymmetry compared to previous mammograms. Nodular asymmetry is typically positioned within the focal asymmetry category, indicating a localized difference in tissue architecture that warrants further investigation.
Common Benign Influences and Changes
The most frequent cause is the natural superimposition of normal fibroglandular tissue, which can create a false appearance of a denser area on a two-dimensional mammogram image. This is often referred to as a summation artifact.
Hormonal fluctuations represent a significant source of variation in breast tissue density and appearance. Changes related to the menstrual cycle, use of hormone replacement therapy, or menopause can alter the amount and distribution of glandular tissue, leading to temporary or persistent asymmetries. Fibrocystic changes, characterized by the presence of cysts and an increase in fibrous connective tissue, can also create a localized, nodular appearance that is denser than surrounding tissue.
Other benign conditions can also manifest as nodular asymmetry. These include fat necrosis, which is the death of fat tissue often following trauma or surgery, resulting in a localized area of thickening. Additionally, conditions like dense stromal fibrosis, where the connective tissue is unusually dense in one area, or pseudoangiomatous stromal hyperplasia (PASH), a non-cancerous overgrowth of cells, can present with this specific mammographic appearance.
The Comprehensive Diagnostic Workup
When nodular asymmetry is identified on a routine screening mammogram, the radiologist will typically assign a BI-RADS Category 0. The initial step in the diagnostic workup is to obtain additional mammographic views of the area in question. These often include spot compression views and magnification views, which allow for a more detailed look at the margins and internal features of the density.
If the asymmetry resolves or dissipates with spot compression, it suggests the finding was simply normal breast tissue overlapping. If the asymmetry persists, the next step is a targeted ultrasound of the area. Ultrasound is crucial because it can distinguish whether the area of density is solid, cystic (fluid-filled), or simply dense glandular tissue, which mammography cannot clearly differentiate.
The findings from the additional imaging are used to assign a final BI-RADS score. If the nodular asymmetry is stable over time and lacks any suspicious features, it may be categorized as BI-RADS Category 3, a “probably benign” finding. This category recommends a short-interval follow-up mammogram, typically in six months, to confirm stability.
If the abnormality has irregular borders, increases in size compared to prior exams (a developing asymmetry), or is associated with other concerning features, it may be upgraded to BI-RADS Category 4. Category 4 findings are considered “suspicious abnormality” and generally recommending a biopsy. A core needle biopsy is the standard procedure, where a small sample of tissue is removed for pathological examination to definitively determine if the cells are benign or malignant.