If you’re one of the 7 percent of people who has gotten a mammogram result that mentions “asymmetry,” it can feel scary to hear. Many people immediately worry that this finding means breast cancer.

In most cases, though, asymmetry on a breast cancer screening mammogram is completely normal and doesn’t lead to a breast cancer diagnosis. Still, doctors take these findings seriously because some types of asymmetry can sometimes be linked to breast cancer.

If your report mentions asymmetric breast tissue, it’s vital to follow through with any recommended testing. Extra imaging is common after screening mammograms. Being called back doesn’t automatically mean something is wrong.

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This article explains what asymmetry on a mammogram means, the different types of asymmetry, when doctors may be more concerned, and what tests may come next.

What Does Asymmetry on a Mammogram Mean?

Breast asymmetry means one breast looks different from the other in size or shape. Breasts are naturally a little different from each other, and many asymmetries are caused by normal breast tissue.

On a mammogram, though, “asymmetry” has a more specific meaning. It refers to an area of breast tissue that looks denser or different compared to the same area in the other breast.

Radiologists (doctors trained to read imaging tests) look closely at these differences. Sometimes asymmetry is simply overlapping tissue captured at a certain angle during the mammogram. Other times, more testing may be needed to see what’s going on.

Asymmetry is one reason people are called back for a diagnostic mammogram.

Different Types of Asymmetry

Radiologists use different terms to describe asymmetry findings. These terms help doctors decide whether more testing is needed.

Asymmetry

A simple “asymmetry” usually means an area of tissue is seen on only one mammogram view instead of several views.

This type of finding is often caused by overlapping normal tissue rather than cancer. In many cases, extra images show nothing concerning.

Focal Asymmetry

A focal asymmetry mammogram finding, often representing dense breast tissue, appears on at least two different mammogram views.

Focal asymmetry on a mammogram report is usually more noticeable than a simple asymmetry, but it still often turns out to be harmless. Radiologists will likely want more images to get a better look at the area.

Global Asymmetry

Global asymmetry means a larger area of one breast has more dense tissue than the matching area in the other breast.

This is usually a normal difference in breast tissue, especially if it has stayed the same over time. Usually, there isn’t a suspicious lump or other concerning change.

Developing Asymmetry

Developing asymmetry is a newer area of breast density that either wasn’t present on earlier mammograms or has become more noticeable over time. This is an uncommon finding on mammograms.

Developing asymmetry concerns doctors more because this kind of changes in breast tissue is linked to cancer in about 10 percent to 15 percent of cases. One study found that developing asymmetry carried enough risk of cancer to justify more testing and often a biopsy.

Still, developing asymmetry doesn’t always mean cancer. Many cases turn out to be noncancerous after follow-up testing. But it’s a finding doctors usually check carefully.

What Can Cause Asymmetry on Mammograms?

There are many reasons why asymmetry may show up on a mammogram.

Sometimes it’s simply because breasts are naturally different. Hormonal changes, aging, pregnancy history, breastfeeding, and weight changes can all affect breast tissue.

Dense breast tissue can also make mammograms harder to read. Dense tissue appears white on a mammogram, and so can tumors, making some areas harder to evaluate.

Other noncancerous causes of asymmetry may include:

• Benign cysts
• Fibrocystic breast changes
• Fibroadenomas (noncancerous breast lumps)
• Scar tissue from surgery or injury
• Overlapping tissue during imaging

In some cases, asymmetry may be an early sign of breast cancer, especially if the area is new, changing, or linked with other concerning findings.

How Often Does Asymmetry Turn Out To Be Cancer?

Most asymmetries found during screening mammograms aren’t cancer.

Being called back after a mammogram is common. According to Johns Hopkins Medicine, fewer than 1 in 10 people called back after a screening mammogram are diagnosed with breast cancer.

The chance of cancer depends on the type of asymmetry and whether it’s stable or changing.

For example:

• A simple asymmetry seen on one view is often caused by overlapping tissue.
• Focal asymmetry may need closer evaluation but is frequently harmless.
• Developing asymmetry, although rare, tends to carry a higher cancer risk than other forms.

Radiologists also look for other suspicious features, including:

• Distortion of breast tissue (irregular appearance of the breast tissue without an evident lump or mass)
• Calcifications (tiny calcium deposits)
• A visible lump, including a cyst or a mass
• Changes compared with older mammograms (new findings)

The more stable an area looks over time, the less concerning it may be.

What Happens After an Asymmetry Mammogram Result?

If asymmetry is found during a screening mammogram, your healthcare team may recommend additional tests to better understand the finding.

This follow-up process with further testing is very common.

Diagnostic Mammogram

A diagnostic mammogram uses extra, more focused images of the breast.

Additional imaging can help show whether the asymmetry is simply overlapping tissue or something that needs more evaluation. Sometimes the extra images completely clear up the concern.

Breast Ultrasound

A breast ultrasound uses sound waves to create images of breast tissue.

Ultrasound is especially helpful for telling the difference between fluid-filled cysts and solid masses. Doctors often use it along with mammograms when checking focal asymmetry on mammogram images.

Breast MRI

Sometimes doctors recommend magnetic resonance imaging (MRI), especially for people with dense breasts or a higher breast cancer risk.

MRI gives very detailed images and may help explain unclear findings from mammograms or ultrasounds.

3D Mammography

Some imaging centers use 3D mammography, also called tomosynthesis.

This technology creates layered breast images that can reduce the effect of overlapping tissue. In some cases, it helps explain asymmetry findings and lowers the need for callbacks.

Breast Biopsy

If imaging still looks suspicious, your doctor may recommend a breast biopsy.

During a biopsy, a small sample of tissue is removed and checked under a microscope for cancer cells.

Although the idea of a biopsy can feel stressful, it’s the best way to get a clear answer. Many breast biopsies turn out to show noncancerous findings.

When Are Doctors More Concerned About Asymmetry?

It’s normal to feel worried after hearing you need more testing. Still, some findings concern doctors more than others.

Doctors may pay closer attention if:

• The asymmetry is new or changing.
• There’s a lump you can feel.
• The area looks suspicious on several imaging views.
• You have skin or nipple changes.
• You have a strong family history of breast cancer.
• Extra imaging shows a solid mass or tissue distortion.

Even then, more testing is needed before doctors can make a diagnosis.

Remember that mammograms are designed to catch even small changes early. Being careful helps doctors find cancer sooner, when treatment often works best.

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