Dense Breasts and Breast Cancer: An Overview

Having dense breasts is one of the risk factors associated with breast cancer. It means you have more glandular and fibrous tissue than fat tissue. Dense tissue makes it harder to detect breast cancer on a mammogram so additional imaging tests may be required.

Most individuals do not know whether or not they have dense breasts because they can't be identified based on their appearance or firmness. Breast density can only be established with a mammogram.

Dense breasts are common and seen in about half of women in the U.S. who have mammograms. Consult your healthcare provider about your breast density and breast cancer risk, and follow any recommendations for additional screenings or imaging tests.

What Are Dense Breasts?

Breasts are composed of fibrous, glandular, and fatty tissue. An individual's breasts are considered dense if they have less fat and more glandular and fibrous tissue than average.

• Fibrous tissue is connective tissue. This is the same type of tissue that makes up ligaments in your body.
• Glandular tissue is the lobes, lobules, and ducts of the breast. Each lobe branches out into lobules containing milk-producing bulbs on their ends. Small ducts connect the lobes, lobules, and bulbs.
• Fatty tissue is the tissue between the fibrous and glandular tissue.

There are four categories used to describe breast density. They go in order from least to most dense.

1. Category A (Fatty): Mostly fatty tissue
2. Category B (Scattered): Scattered areas of the denser glandular and fibrous tissues
3. Category C (Heterogeneous): More areas of the denser glandular and fibrous tissues
4. Category D (Extremely Dense): Almost all glandular and fibrous tissue with little to no fatty tissue

Generally, categories C and D are considered dense breasts.

Categories B and C, scattered and heterogeneously dense breasts, are the most common mammogram findings. Roughly 40% of females have scattered breasts; 40% have heterogeneously dense breasts; approximately 10% have fatty breasts; and 10% have extremely dense breasts.

Causes

Dense breasts are a common mammogram finding. Nearly half of all females 40 years and older have dense breasts detected on their mammogram.

Four main factors can impact whether or not you have dense breasts:

1. Genetics: Dense breasts are often an inherited trait.
2. Menopausal hormone therapy: This is also known as hormone replacement therapy (HRT).
3. Low Body Mass Index (BMI): Less body fat increases the possibility of more dense breast tissue.
4. Younger age: Breast tissue tends to become less dense over time.

Protective factors against dense breasts include older age and past pregnancies.

Some data suggest oral contraceptives contribute to increased breast density. If oral contraceptives are started at an earlier age, there may be an association with increased breast density.

Dense Breasts and Increased Risk of Cancer

Dense breasts are associated with an increased risk of breast cancer. This risk is in addition to the fact that dense breasts are more challenging to read on mammograms. One theory for the increased risk is that dense breasts contain more cells than non-dense breasts. So, there are more opportunities for abnormal cells to develop.

Individuals with extremely dense breasts are four to five times more likely than those with mostly fatty tissue breasts to develop breast cancer. Individuals with heterogeneously dense breasts are also more likely to develop breast cancer.

There are several different types of breast cancer, and more research is needed on the types of breast cancer associated with dense breasts. However, females under age 55 with dense breasts are more likely to develop estrogen receptor-negative tumors. Larger tumors and lymph node involvement are also seen in denser breasts.

The Breast Cancer Surveillance Consortium (BCSC) Risk Calculator is a tool used to assess five- and 10-year breast cancer risk based on age, race, ethnicity, family history of breast cancer, history of a benign breast biopsy, and breast density. While dense breasts are part of this calculation, they are not the strongest risk factor.

Mammogram Accuracy

The increased risk of breast cancer in dense breasts is particularly concerning because dense breast tissue can lead to false-negative mammograms—that is, a missed diagnosis. The sensitivity (ability to accurately identify a tumor) is much lower in those with heterogeneous and extremely dense breasts.

Dense breasts are characterized by white areas on a mammogram—and so is breast cancer. Dense breast tissue can obscure tumors, which is why the interpretation of a mammogram can be challenging in females with dense breasts.

However, mammograms are still considered the screening tool of choice for individuals who have dense breasts because they identify most breast cancers. They are believed to have more accuracy than other diagnostic tests. More research is needed to understand if additional imaging, such as MRIs or ultrasounds, are beneficial as screening tools.

Additional Breast Cancer Screening Tools

Additional screening tools, such as breast cancer screening ultrasounds and MRIs, may pick up a cancer that is too challenging to detect on a mammogram because of dense breast tissue.

Your provider can help you assess your overall breast cancer risk to determine if additional screening is needed.

Screening Ultrasound

For women who have dense breasts, the combination of mammography and a screening breast ultrasound may increase the detection of breast cancer (but with more false positives).

Abbreviated MRI

Research suggests that adding abbreviated breast magnetic resonance imaging (MRI) to mammography is more likely to detect breast cancers, especially aggressive ones.

While conventional breast MRI is the most sensitive measure to detect the disease, it is costly relative to mammography. It is currently only recommended for people who have a high lifetime risk of developing breast cancer.

An abbreviated breast MRI takes only about 10 minutes to perform and significantly increases the chance of finding breast cancer that is present.

With regard to cost, it is comparable to the cost of mammography, thus offering an option for women who have an elevated risk of breast cancer due to having dense breasts but who do not meet the criteria for conventional breast MRI screening.

Since abbreviated MRIs are a relatively newer protocol, not all radiology departments can do this screening.

Genetic Testing

Various genes are associated with a higher lifetime risk of breast cancer. Two genes associated with a significantly higher breast cancer risk are the BRCA1 and BRCA2 genes. Your healthcare provider can help you determine if genetic testing is appropriate and order genetic tests.

Breast Density Reporting

Concern over the increased risk for breast cancer in women with dense breasts has resulted in legislation. The legislation mandates healthcare providers inform patients if they have dense breasts and discuss additional imaging options following a normal mammogram.

The U.S. Food and Drug Administration (FDA) regulates mammogram facilities. They require that mammogram reports include an assessment of breast density along with language explaining the impact of breast density on the accuracy of the report and the risk of breast cancer.

Reports are required to indicate if breasts are "dense" or "not dense" and use specific wording to explain to the patient what dense and not dense breasts mean.

Summary

Dense breasts are common and seen in about half of U.S. women who have mammograms. They are a risk factor for breast cancer but other factors, including inherited gene mutations and a family history of breast cancer, are considered bigger risk factors.

Dense breasts can obscure breast cancer detection on mammograms. If you have dense breasts, discuss your overall breast cancer risk with your healthcare provider. You may need additional imaging tests in addition to an annual mammogram.

Perform a monthly breast self-examination and be attentive to changes in your breasts. Contact your healthcare provider right away if you notice any changes or pain.