Focus on Imaging: Is a mammogram enough to find cancer in dense breasts?

The American College of Radiology recommends mammograms yearly for all women ages 40-75 for the early detection of breast cancer. And, says Dr. Emily McCarty, medical director at Women’s Health, “if there is a higher risk condition, such as a close family member who had breast cancer at a younger age, mammograms should generally begin 10 years prior to the age of diagnosis of that relative.” Since 1990, with the adoption of routine breast cancer screening, U.S. mortality from breast cancer has decreased by a whopping 40 percent.

But the accuracy of mammography for women with the common makeup of dense breasts is not as successful. Around half of all women have dense breasts, but only since 2023 has it been required to be inform those women about their breast density.

Breasts are composed primarily of three tissue types, says Valley View Radiologist Dr. David Breland: glands that make milk, fibrous connective tissue and fat. The more glands and connective tissue, the denser the breast. As a part of routine screening, radiologists assess breast density subjectively according to the following scale:

a. Almost entirely fatty

b. Scattered areas of fibroglandular density

c. Heterogeneously dense, which may obscure detection of small masses

d. Extremely dense, which lowers the sensitivity of mammography

Categories C and D are considered dense breasts. FDA guidance now requires that routine follow-up communication for screening mammograms includes information about a woman’s breast density and a recommendation that they talk to their health care provider regarding additional screening options and their individual risk for breast cancer.

One of the difficulties with dense breasts is that on a mammogram, even a 3-D mammogram like those routinely used at Valley View Imaging, the glandular and connective tissue has a shade similar to the abnormal tissue in cancers and also may obscure tiny mineral deposits called calcifications which are in some cases an important clue to the presence of a cancer or precancerous lesion. In a fatty breast, mammograms are up to 98 percent accurate; in an extremely dense breast, the accuracy can be as low as 40 percent.

Additionally, higher breast density has been found to increase a woman’s risk of breast cancer. In fact, those with extremely dense breasts may be four to five times more likely to develop breast cancer than women with very fatty breasts, according to Dr. Breland.

“Dense breasts have a double whammy effect – cancer is more likely to be there, but it is also more likely to be undetectable using only a mammogram” he says.

To more accurately screen women with dense breasts, Dr. Breland recommends adding whole breast Automated Breast Ultrasound or ABUS to mammography. ABUS is a painless 15-minute scan performed using a specialized ultrasound machine.

“By offering ABUS in addition to mammography for our patients with dense breast tissue, we are improving detection for small cancers that cannot be seen on a mammogram alone in these women,” he says. “ABUS has been an integral part of our breast screening program for about three years now.”

With ABUS available at Valley View, radiologists like Dr. Breland can review hundreds of breast tissue image “slices” to look at layers of dense tissue and find abnormalities which may have been undetectable on a mammogram. Research shows that ABUS technology as an adjunct to mammography has the potential to increase the cancer detection rate in women at average risk by 35.7 percent compared to mammography alone.

It is normal for up to ten percent of women undergoing screening with either mammography or ABUS to be asked to return for additional imaging, usually with more mammograms and/or a handheld ultrasound examination which takes about 30 minutes for a more detailed evaluation. In fewer than half of those cases it is necessary to perform an image-guided biopsy to obtain a tissue sample for definitive diagnosis.

Women with additional risk factors may benefit from a supplemental MRI screening rather than ultrasound. MRI has the highest sensitivity for detecting breast cancer of the tests currently available. According to New York Magazine, “a study in The Lancet that examined the results of cancer screens for more than 7,000 women and found that mammograms missed 48 percent of ductal carcinoma in situ (the earliest form of breast cancer), while MRIs missed only two percent. And in dense tissue, tumors and masses will show up on a breast MRI as distinctly white against a black background.”

“An MRI has the best results, but for a variety of reasons it is not ideal as a screening test for most patients and is mainly used for patients known to be at high risk,” explains Dr. Breland. “They take longer, are far more expensive, and require temporarily placing an IV catheter so that contrast can be injected into the patient’s bloodstream.”

Looking ahead, there is research around the concept of “abbreviate MRIs,” where radiologists will one day be able to routinely screen women for breast cancer using a non-contrast MRI scan that takes about as much time as a mammogram. While this is a hopeful development for quicker and more accurate scans for women with dense breasts, it is not currently available, but a possibility for the future.

“All women should feel empowered to perform self-breast examination regardless of their breast density,” says Dr. McCarty. “We recommend a once a month exam, ideally the week after a menstrual period, if a woman is still having one. The advantage to regular self-exam is the ability to detect differences month to month.”