Mammography provides a detailed X-ray view of the breast tissue. When reviewing these images, radiologists look for any deviation from a typical appearance, and a common finding is “asymmetry.” This term indicates an area in one breast that looks different from the corresponding area in the other breast. While this finding often prompts a request for additional imaging, the vast majority of asymmetries are not signs of cancer.
Defining Mammographic Asymmetry
Mammographic asymmetry refers specifically to an area of increased fibroglandular density in one breast that lacks the distinct borders and three-dimensional shape required to be classified as a mass. Essentially, the radiologist observes a patch of tissue that appears denser than the same location in the opposite breast. This finding focuses entirely on the internal tissue composition, distinguishing it from normal anatomical variations in breast size or shape.
This increased density is often due to the overlapping of normal breast tissue within the compressed image, sometimes referred to as a summation artifact. An asymmetry can be caused by a variety of conditions, ranging from simple tissue overlap to a subtle underlying abnormality. Because it represents an uncertain finding, an asymmetry detected on a screening mammogram frequently leads to a request for further evaluation.
Categorizing Different Types of Asymmetry
Radiologists use precise terminology based on the Breast Imaging Reporting and Data System (BI-RADS) lexicon to classify asymmetries and determine the likelihood of a benign versus a malignant cause. The most basic finding is “asymmetry,” which describes an area of increased density visible on only a single mammographic projection, such as the craniocaudal view. Since this finding disappears on other views, it most often represents a benign summation of normal tissue.
The classification shifts to “focal asymmetry” when the increased density is clearly visible on two different mammographic views, such as both the craniocaudal and the mediolateral oblique projections. A focal asymmetry is typically a small, localized area involving less than a full quadrant of the breast. Although a focal asymmetry warrants additional workup, it is still frequently found to be benign.
A “global asymmetry,” by contrast, involves a large area of increased density, often encompassing more than an entire quadrant of the breast. This type of finding is usually considered a normal anatomical variant, reflecting a natural difference in the total amount of fibroglandular tissue between the two breasts. Global asymmetries are rarely associated with malignancy unless they are accompanied by other suspicious features.
The most concerning classification is “developing asymmetry,” defined as a focal asymmetry that is either new or has become larger and more conspicuous compared to previous mammograms. Since normal breast tissue typically becomes less dense over time, a new or growing density raises more suspicion than a stable finding. Developing asymmetries have a higher probability of representing a malignancy, such as invasive lobular carcinoma, and they require thorough investigation.
Benign and Malignant Causes of Asymmetry
The underlying reasons for a mammographic asymmetry are diverse, and most stem from non-cancerous changes within the breast tissue. The most frequent cause is the overlap of normal fibroglandular tissue, which creates a perceived density that is not a discrete lesion. Other common benign causes include localized hormonal effects, which can temporarily increase tissue volume or density in one area.
Changes related to prior trauma or intervention can also manifest as an asymmetry. For instance, the formation of scar tissue after a biopsy or surgery, or a condition like fat necrosis, can result in an area of localized thickening. Benign conditions such as cysts or localized areas of dense stromal fibrosis may similarly present as an asymmetric density.
While the majority of asymmetries are benign, the finding can, in a small percentage of cases, be the first indicator of breast cancer. Malignancies like invasive ductal carcinoma or invasive lobular carcinoma can sometimes present initially as an asymmetric density rather than a well-defined mass. The presence of an asymmetry necessitates further evaluation to distinguish a benign finding from a subtle malignant process.
The Diagnostic Workup and Follow-Up Process
When a screening mammogram detects an asymmetry, the result is categorized as BI-RADS 0, meaning the finding is incomplete and requires additional imaging evaluation. The initial step is a targeted diagnostic mammogram, which includes specialized views to better characterize the area of concern. This typically involves spot compression views, where concentrated pressure is applied to the specific area. This focused compression helps spread out the tissue, often causing a summation artifact to disappear and confirming it as benign overlapping tissue.
If the asymmetry persists on the spot compression views, magnification views may be utilized to look for subtle associated features, such as suspicious microcalcifications. Following the additional mammographic images, a breast ultrasound is performed on the corresponding area. Ultrasound is effective at differentiating between a solid mass and a fluid-filled cyst, which may not be possible on the mammogram alone. If the ultrasound identifies a simple cyst or another benign feature, the finding is downgraded to BI-RADS 2 (benign).
If the asymmetry remains stable after the diagnostic workup, without suspicious features like architectural distortion or calcifications, it may be classified as BI-RADS 3. This indicates a probably benign finding with a less than two percent chance of malignancy. The recommended course of action is a short-term follow-up mammogram, typically in six months, to confirm stability over time.
If the asymmetry is developing, or if it has suspicious features such as irregular margins or associated architectural distortion, it will be classified as BI-RADS 4 (suspicious abnormality). A BI-RADS 4 classification necessitates a tissue sample, most often through a core needle biopsy, to determine the cellular nature of the density.