Your report from a mammogram may include notes about asymmetry. Asymmetric breast tissue refers to differences between one breast and the other. The report may mention differences in:

  • tissue volume
  • structure
  • density

Asymmetric breasts are fairly typical, and there are several kinds of breast asymmetries. In most cases, differences between your breasts are not a cause for concern. But large differences can sometimes be a sign of breast cancer.

Focal asymmetry refers to localized areas in one breast that look different from the corresponding areas of the other breast. If your mammogram shows new areas of focal asymmetry during screening, a doctor may recommend you come back for further testing.

Mammogram callbacks can be scary and upsetting. But focal asymmetry rarely predicts the occurrence of breast cancer.

In this article, we’ll look at what might cause focal asymmetry and what to do if it turns out to be cancer.

Think of your breast in four quadrants, with the nipple at the center. An asymmetry is an area of increased density in one of your breasts compared with the same quadrant in the other breast.

A focal asymmetry must be smaller than a single quadrant in any area of the breast. It must also appear on two or more views (angles) of a mammogram for a radiologist to consider it a focal asymmetry.

Focal asymmetry does not mean you have cancer. While some cases may be due to a malignant mass, it is most often due to other causes. Still, it may warrant further examination.

On a screening mammogram, focal asymmetries usually lack the ominous borders that raise suspicion for a cancerous mass. But these borders may look different on further diagnostic tests. Even so, the chances of a focal asymmetry turning out to be cancer are low.

Radiologists use the Breast Imaging Reporting and Database System (BI-RADS) to assess your risk of cancer. They classify focal asymmetry as BI-RADS category 3, meaning that it is probably noncancerous. This suggests a 2% or less chance of cancer.

The chance of cancer may be higher if the asymmetry contains suspicious characteristics. The radiologist will look for microcalcifications — calcium deposits that look like white specks on a mammogram. They’ll also look for alterations in breast tissue shape, such as an indentation or pulling.

The radiologist reading your mammogram will compare it to earlier mammogram films, if possible. If it’s the first time an asymmetry appears, or if it changes from previous films, they may consider it a developing asymmetry. These have a slightly higher risk — 12% — of cancer.

If you have a developing asymmetry, a doctor may recommend further testing. These tests may include a diagnostic mammogram or a breast ultrasound.

After additional tests, most focal asymmetries turn out to be dense fibroglandular tissue that is most likely noncancerous. A doctor may recommend more testing in 6 months to check for changes that might indicate breast cancer.

Other types of breast asymmetries

There are four categories of breast asymmetry. In addition to focal asymmetry, the other three are:

  • Global asymmetry: This is larger than focal asymmetry. Still, it can turn out to be noncancerous. If it is cancerous, there will usually be a mass in your breast that you can feel.
  • Developing asymmetry: This is a focal asymmetry that becomes larger or denser in subsequent mammograms over the course of months or years. This change is often suspicious but doesn’t always turn out to be cancer. Still, it warrants further testing.
  • One-view (single-view) asymmetry: This is a focal asymmetry doctors see on only one mammogram view. Further testing shows that few one-view asymmetries turn out to be cancerous. Most one-view asymmetries are summation artifacts, a type of distortion to the image.

Your breasts, just like your extremities, may be difficult to tell apart. However, they’re rarely identical or completely symmetrical. Small differences are typical and expected. Slight internal asymmetries may not be visible to the eye, but you can see them on imaging tests.

Focal asymmetry in breast tissue is common. It can be due to natural differences in breast volume, form, and size. In some instances, a developing cancer may be the cause.

Focal asymmetry may also be due to problems with mammogram technology. The superimposition of regular breast tissue on film can look like an area of increased density, or mimic the appearance of a lesion, where none exists. Doctors refer to this as a summation artifact.

If your screening mammogram shows focal asymmetry for the first time, a doctor may recommend further testing. They’ll consider your breast density and breast cancer risk factors in determining which tests you need. In most cases, they will eventually rule out breast cancer after these tests.

The next step may be a diagnostic mammogram. Like screening mammograms, diagnostic mammograms are X-rays of the breast. Diagnostic mammograms focus on specific, suspicious areas that doctors identify on your screening mammograms. They show more detailed images.

You may also get a breast ultrasound. Breast ultrasounds do not screen for breast cancer because they don’t always pick up images of microcalcifications. They are, however, beneficial for viewing inside dense breast tissue.

If doctors still suspect cancer, they may recommend an MRI scan or a biopsy.

Breast MRIs are imaging tests. They allow doctors to view breast tissue in people with very dense breasts and those at high risk of breast cancer. If a doctor does find cancer, an MRI scan can also help determine the extent of its spread, if any.

A biopsy is the only way to definitively diagnose breast cancer. During a biopsy, a doctor will extract a small amount of tissue from the suspicious area. They’ll send the tissue sample to a laboratory, where lab technicians will check for cancerous cells.

When you need additional testing

Don’t panic if you need additional testing after a screening mammogram. Mammogram callbacks are common and don’t mean a doctor has found cancer. In fact, fewer than 1 in 10 people called back for more testing have cancer.

People may get mammogram callbacks because doctors need to:

  • retake pictures because the films are unclear
  • recheck a small area of breast tissue they may have missed
  • reexamine a suspicious area, such as a mass or asymmetry

Most often, suspicious masses turn out to be noncancerous, fluid-filled cysts or areas of dense, regular breast tissue.

No one wants to hear they have cancer. But it may help to remember that treatments, and success rates, have improved significantly over the years.

If a doctor finds cancer, they’ll refer you to a breast cancer specialist (oncologist). The specialist will perform tests to determine the size of the tumor and the stage of cancer you have. This will help determine your treatment plan.

All of this can be overwhelming. Ask your oncologist as many questions as you need to. You can also go to another oncologist for a second opinion. If you decide to do so, bring all of your test results, and a list of questions, with you to your appointment.

It may also help to find a cancer support group, or to work with a mental health professional who specializes in breast cancer. The American Cancer Society and Susan G. Komen are good places to start. Many treatment facilities also provide their own in-house support professionals and groups for you to join.

Questions for your doctor

It’s important to keep yourself informed. You may wish to bring a “second set of ears,” such as a friend or relative to your doctor’s appointment, who can advocate for you. You may also wish to record your conversation, with your doctor’s permission.

Questions to ask include:

  • What type of breast cancer do I have?
  • Where is the tumor located?
  • What size is the tumor?
  • Has my cancer spread, and if so, where?
  • What stage and grade is my cancer?
  • What treatments can I expect?
  • What are the potential side effects of treatment?
  • How should I prepare for my treatments?
  • What is the average survival rate for people with this type of cancer?
  • Are there clinical trials available that I should consider?
  • Is financial help available for treatment if I need it?

Focal asymmetry refers to a difference in appearance between each breast that doctors observe on a mammogram. These differences are smaller and usually not cancerous. Even so, a doctor may recommend further testing.