We often think of breasts as mirrors of each other - and to a degree, that’s correct. Symmetry can be a beautiful thing, and one of the things radiologists assess when we are looking at your mammogram. But just as your face and your feet may be slightly different from side to side, so can your breasts. At least a quarter of the population has two different sized breasts. In other words, asymmetry can be quite normal.
There are different kinds of asymmetries, from difference in size to tissue density. Both are features we look at on your breast imaging study. On a mammogram, an asymmetry typically means there’s more tissue, or white stuff on the mammogram, in one area than on the opposite side.
When asymmetry occurs, it leads to a question: is this normal for that person? The answer is something a radiologist will try to uncover. When a mammogram shows an asymmetry, the first step is to see if this has been present on any other studies. Has the asymmetry always been there? If so, it is likely a normal degree of side-to-side difference.
If it’s new or we don't have any other studies to compare, the next step is to figure out what is making the area look different. This may require additional imaging, either with mammography and/or possibly breast ultrasound. The work-up tests may show normal looking tissue in the area. In this case we may recomomend a short- term follow-up study (BI-RADS 3) if we don't have access to older studies. The additional tests may also uncover a mass such as a breast cyst. Benign, noncancerous masses can appear as a focal asymmetry. Breast cancer can present either as an area of focal asymmetry or when advanced can even present as a new asymmetry in breast size. This is why you should always talk to your doctor if you notice an unexplained change in the size of a breast.
If your mammogram report talks about asymmetry or if you need a follow-up study due to asymmetry, there’s no need to worry. You may simply have more tissue in one breast than another (global asymmetry), or in one spot (focal nodular asymmetry). By using additional mammogram images, comparing prior studies to current ones, or by using different modalities like ultrasound, a radiologist can usually determine the cause of the finding. If you have questions about your breasts or your breast imaging reports, don't hesitate to ask. Knowledge and following screening guidelines are the keys to healthy breasts!