What are dense breasts, and why is it important for women to know if they have them?

September 24, 2020
Dr. Rebecca Leddy points to a mammogram image on a computer screen
Dr. Rebecca Leddy stresses the importance for women to know if they have dense breasts, which can obscure mammogram findings. Cancer and normal dense tissue both appear white on mammograms. Photo by Dawn Brazell

Early-stage breast cancer can be cured in most women. The key is catching it early, which can be difficult in women who have a type of tissue called dense breasts.

MUSC Hollings Cancer Center radiologist Rebecca J. Leddy, M.D., said it’s important that women understand how dense breasts can affect their screening results and know how to advocate for themselves. Here, she explains the condition and what she wished women knew about screenings.

What does it mean to have dense breasts?

The breasts are made up of a variety of tissue and are considered dense when there is more fibrous and glandular tissue than fat. Breast density is not determined by how a breast looks or feels, but rather through breast imaging, specifically from radiologists interpreting mammograms. Having dense breasts is normal, with more than half of women under the age of 50 having dense breasts, 40% of women in their 50s and 25% of women age 60 and older.

How can having dense breasts affect screening for breast cancer?

Dense breasts can obscure or hide findings on the mammogram, which is often called the masking effect. On a mammogram, cancer is white; however, normal dense tissue also appears white. If a cancer (white) develops in an area of normal dense (white) tissue, it can be harder or sometimes impossible to see it on the mammogram. If a cancer (white) develops in an area of fat (black or dark gray), it is usually easier to detect.

If a person’s breasts are dense on the mammogram, the ability to see cancer on mammography decreases. However, many cancers are seen on mammograms even if you have dense breast tissue. Therefore, it is still necessary to get an annual mammogram each year, starting at age 40 in average-risk patients, regardless of density. Multiple studies have proved that the early detection of cancers by mammography reduces the risk of death due to breast cancer for women aged 40 to 74 years.

What are your screening suggestions for women with dense breasts?

3D mammography, or digital breast tomosynthesis, has been shown to increase cancer detection in all types of breasts, especially in women with dense breasts. All women, irrespective of breast density, should undergo 2D or 3D annual screening mammography starting at age 40 for those at average risk for breast cancer and potentially earlier if they are high risk for breast cancer. Depending on your risk level for breast cancer and your breast density, additional supplemental screening with ultrasound or MRI may be recommended in addition to your routine 2D or 3D/tomosynthesis mammogram.

Studies have shown that ultrasound and MRI can help find breast cancers that can't be seen on a mammogram and should be used in addition to the mammogram. Women with an increased lifetime risk of breast cancer, greater than 20%, should undergo screening MRI in addition to mammography regardless of breast density. For those women who are not high risk to qualify for screening MRI, but have dense breasts, having supplemental screening ultrasound in addition to mammography is an option that can be used. The caveat is that although extra testing after mammogram may find more cancers, it can also result in added testing and more biopsies. At MUSC, a preliminary assessment of a patient’s lifetime risk of breast cancer is included in her mammogram report to evaluate which patients should then undergo more accurate assessment and who may need additional screening.

What factors play a role in having this condition? 

Breast density is affected by many factors such as genetics, age, hormones, body weight and any systemic health problems. In general, breasts tend to become less dense as women get older, especially after menopause when the glandular tissue atrophies and the breast may appear to have more fat. An increase in breast density often is seen in women taking exogenous hormones such as hormonal replacement therapy. Having conditions that change the fluid status of the body, such as heart failure or renal failure, can also change the person’s breast density.

Weight changes can also affect breast density in that if a patient loses a lot of weight, her breasts may appear denser due to the relative loss of fat. While losing weight may cause the breast to become denser on a mammogram, these are healthy life choices to maintain as obesity, excessive alcohol intake and smoking can increase your risk of breast cancer regardless. Regarding diet, a study published in Obstetrics and Gynecology in September of 2016 suggested that a diet of high fat dairy, high processed meats and refined grains can increase breast density in overweight-obese women. Other studies have reported diets high in sugar and alcohol consumption are associated with increased breast density. Given the variability of these factors, breast density reporting can fluctuate year to year. For example, if your weight fluctuates, one year a woman may be told she has dense breasts, and the next year that they are not.

What are the four categories, and why should women know where they fall?

The Breast Imaging Reporting and Database System (BIRADS) is a reporting database for breast imaging supported by the American College of Radiology and is set up in four categories from least to most dense. Most women will fall into the scattered fibroglandular (40%) or heterogeneously dense categories (40%), 10% of women will have extremely dense breasts, and 10% will have predominately fatty replaced breasts. Therefore, half of women will have dense breasts and half not. Women should be aware of their breast density to know their risk factors for breast cancer and to determine with their physicians a need for any additional screening imaging in addition to mammography.

How does a specialized radiology team help?

The MUSC breast imaging team comprises radiologists who are fellowship trained in breast imaging, meaning they had additional training specifically dedicated to breast imaging. In addition, the MUSC team of breast radiologists’ primary focus is breast imaging and not a general radiology practice. The importance of MUSC having radiologists dedicated to breast imaging was emphasized in a study published by Annals of Surgical Oncology in 2018. In the study, it was reported that 23% of the patients seen for second opinions at the MUSC Multi-Disciplinary Tumor Board had additional cancers diagnosed based on reinterpretations by MUSC breast imaging fellowship-trained radiologists, and 43% had a change in management, mostly related to additional radiologic recommendations by our radiology team. The study concluded that review by breast fellowship-trained radiologists who are part of a tumor board is one of the areas in which the most benefit can be gained in breast cancer second opinions.

Are dense breasts a risk factor for breast cancer?

Risk for developing breast cancer is influenced by a combination of many different factors including age, family history of cancer (particularly breast and/or ovarian cancer), genetic mutations, prior atypical breast biopsies, history of radiation to chest, alcohol use, exogenous hormone use and obesity, to name a few. Several studies found that the risk of breast cancer increases with increasing breast density in that women with dense breasts have a 4- to 6-fold increased risk of breast cancer compared to women with less dense breasts. The more active glandular tissue or more fibrous tissue a person has (dense breasts), the greater the risk that the tissue can change and become cancerous.

However, most women who develop breast cancer have no additional risk factors other than being female and aging. Maintaining a healthy lifestyle with limiting alcohol consumption, eating healthy, exercising, controlling your weight, never smoking — or quitting if you do smoke — have all been correlated with a reduced risk of developing breast cancer. Early detection with mammography is one of the most important factors to reduce risk of death from breast cancer.

How are researchers and radiologists making progress in shedding light on this topic?

The education of patients and their ordering providers about breast density and the increased risk of breast cancer is one step. Many states, including South Carolina, have a breast density notification law. This law states that patients should be made aware of their density seen on mammogram. Several recent studies have been published, reporting on the effectiveness of these laws. They found that although patients found out they had dense breasts in states with breast density notification laws, there was no increase in understanding the implications of having dense breasts. In addition, several studies have shown no increase in the use of screening ultrasounds. Education of the implications of breast density and options for imaging dense breasts is important.

Radiologists are also looking into various imaging modalities and the ability to detect cancer in dense breasts. Some of these newer imaging modalities include a shortened version of breast MRI called abbreviated breast MRI and using that in women who have dense breasts but do not fall into a high-risk category. Currently breast MRI is only utilized in addition to mammography for high risk women. According to a study published in February of 2020 in the Journal of the American Medical Association, abbreviated breast MRI detected significantly more cancers than mammography alone in average-risk women with dense breast tissue. Other researchers are evaluating use of medications such as Tamoxifen or Aromatase Inhibitors to decrease breast density and decrease risk.