A mammogram showing asymmetry means one area of breast tissue appears different in density or structure compared to the corresponding area in the other breast, or sometimes within the same breast. This finding is quite common, occurring in about 3% of mammograms. It is important to remember that such a finding usually indicates a benign condition. An asymmetry on a mammogram simply flags an area that requires further investigation by a radiologist, rather than automatically indicating breast cancer.
Types of Asymmetry on a Mammogram
Radiologists classify mammographic asymmetries into types based on their appearance and the number of mammogram views on which they are visible. A general “asymmetry” is a unilateral increased fibroglandular density that is seen on only one projection. These findings represent overlapping normal breast tissue.
A focal asymmetry is a finding seen on two projections. It involves less than one breast quadrant and lacks convex borders, meaning it can appear interspersed with fat. Global asymmetry describes an increase in breast tissue density that involves more than one quadrant and is also visible on two projections.
Developing asymmetry is a noteworthy type, defined as a focal asymmetry that is new, larger, or more noticeable when compared to previous mammogram images. This can make developing asymmetries more concerning, potentially warranting additional follow-up. Architectural distortion is a related finding where the normal arrangement of breast tissue appears disrupted, like a change in the shape or orientation of ducts, without a clear mass.
Common Reasons for Asymmetry
The majority of asymmetry findings stem from benign causes. One frequent reason is normal variations in breast tissue density, where the distribution of fibrous and glandular tissue naturally differs between the breasts or within areas of the same breast. This can be due to overlapping breast tissue.
Hormonal changes also play a role in temporary breast tissue variations. Fluctuations during the menstrual cycle, use of hormonal replacement therapy, or oral contraceptives can lead to changes in breast size, volume, and density, which may appear as asymmetry on imaging. Benign conditions such as cysts, which are fluid-filled sacs, or fibrocystic changes, involving fluid-filled cysts and scar tissue, are common causes of asymmetry.
Previous breast surgery or trauma can also result in asymmetry due to scar tissue formation or fat necrosis, a condition where damaged fat tissue appears as a dense area. Other benign causes include inflammation, such as mastitis, or pseudoangiomatous stromal hyperplasia (PASH), which is a benign overgrowth of cells. Even weight fluctuations can influence breast tissue distribution and density, contributing to observed asymmetries.
What Happens After an Asymmetry Finding?
When an asymmetry is detected on a screening mammogram, the next step is typically a “callback” for further diagnostic evaluation. This is a common occurrence, with about 1 in 10 screening mammograms resulting in a callback, and approximately 90% of these callbacks ultimately reveal nothing serious. The initial asymmetry finding is categorized as BI-RADS 0, meaning incomplete, and additional imaging is needed to clarify it.
The diagnostic workup usually begins with additional mammogram views, such as spot compression and magnification views. Spot compression applies pressure to a smaller, specific area to spread out tissue and get a clearer image, while magnification views provide a more detailed look at suspicious areas. A breast ultrasound is also commonly performed, using sound waves to create images and determine if an area of concern is a solid mass or a fluid-filled cyst.
A breast MRI may be recommended, particularly if initial tests are inconclusive or if there are other risk factors. If the asymmetry persists or shows suspicious features after these additional imaging tests, a biopsy might be suggested. A biopsy involves taking a small sample of breast tissue, often with a fine needle, to be examined under a microscope by a pathologist to definitively determine if cancer cells are present.
When Asymmetry Might Be a Concern
While most asymmetries are benign, certain characteristics or changes can elevate the level of suspicion, necessitating closer scrutiny. A new asymmetry or one that has increased in size or conspicuity since a previous mammogram, known as a developing asymmetry, is often considered more concerning. This type of asymmetry has a higher likelihood of being associated with malignancy compared to other asymmetry types.
Architectural distortion within the asymmetric area raises concern. This refers to a disruption of the normal breast tissue pattern without a defined mass, which can sometimes be a subtle sign of malignancy. If microcalcifications (tiny calcium deposits) are found within the asymmetry, especially if they are new or have a suspicious pattern, further investigation is warranted.
A palpable mass associated with the asymmetry is another feature that prompts greater vigilance. Even with these features, asymmetry only indicates a need for further diagnostic evaluation. These findings do not confirm malignancy but rather guide radiologists to perform additional tests, such as targeted ultrasound or biopsy, to arrive at a definitive diagnosis.