Regular mammograms can detect breast cancer when it is too small to be felt as a lump. Detecting cancer at this early stage — before it has the opportunity to spread — allows for simpler, more effective treatment with a higher likelihood of success.

In addition to identifying cancer, mammograms can also determine whether a person has dense breast tissue, which may indicate the need for additional screening.

In simple terms, breast density refers to how breast tissue appears on a mammogram. Having dense breasts is common and entirely normal — nearly half of women aged 40 and older have dense breast tissue.

However, knowing one’s breast density is important because it can influence both screening recommendations and overall breast health. In some cases, mammograms alone may not be sufficient to detect breast cancer, making awareness and supplemental screening critical for early detection.

What Is Dense Breast Tissue?

Breasts are composed of two main types of tissue:

• Fibroglandular tissue, which includes:
  • Glandular tissue (milk glands and ducts)
  • Fibrous tissue that supports and holds the breast structure in place
• Fatty tissue, which surrounds and cushions the fibroglandular tissue

Each individual’s breasts contain different proportions of fatty and fibroglandular tissue. Based on these proportions, breast density is classified into four categories:

1. Mostly fatty tissue – the least dense type
2. Scattered fibroglandular density – primarily fatty with some dense areas
3. Heterogeneously dense – mostly dense tissue with some fatty areas
4. Extremely dense – predominantly dense tissue throughout the breast

Individuals with heterogeneously dense or extremely dense breasts are considered to have dense breast tissue, meaning that more than half of the breast consists of glandular and fibrous tissue rather than fat.

Breast density cannot be determined by appearance or feel; it can only be identified through imaging tests, such as a mammogram.

What Causes Dense Breast Tissue?

Several factors can increase the likelihood of having dense breast tissue, including:

• A genetic predisposition
• Being younger than age 50
• Having a lower body weight or body mass index (BMI)
• Taking hormone replacement therapy
• Being pregnant or breastfeeding

Breast density can change over time, meaning that a person who currently has dense breast tissue may not have it later in life.

Hormonal fluctuations also play a significant role in breast density. Changes that occur during the menstrual cycle, pregnancy, and menopause can all affect the composition of breast tissue. Pregnancy often results in increased breast density, while postmenopausal women generally experience a decrease in density due to hormonal changes.

What Does Having Dense Breast Tissue Mean?

Having dense breast tissue is associated with a higher risk of developing breast cancer. For this reason, regular mammograms are especially important. In some cases, healthcare providers may also recommend supplemental screening methods to ensure a more comprehensive evaluation.

Because both dense breast tissue and tumors appear white on 2D mammograms, cancers can sometimes go undetected, as the dense tissue may obscure abnormalities. In contrast, fatty breast tissue appears darker on a mammogram, making it easier to identify potential issues.

To improve detection, doctors may order additional imaging tests that provide a more detailed view of the breast tissue, including:

• 3D Mammogram (Tomosynthesis): An advanced imaging technique that uses X-rays from multiple angles to create a detailed, three-dimensional image of the breast.
• Breast Ultrasound: A noninvasive test that uses sound waves to produce 3D images of the breast, helping distinguish between solid masses and fluid-filled cysts.
• Breast MRI: A test that uses magnetic fields to generate detailed 3D images of the breast and can identify smaller cancers that might not be visible on a mammogram. Breast MRIs are typically recommended for individuals with dense breast tissue and a higher risk of breast cancer.

In most cases, follow-up imaging after a standard screening mammogram reveals no abnormalities. However, undergoing these additional tests is important, as early detection of breast cancer leads to simpler, more effective treatment and better overall outcomes.

Dense Breast Tissue: Knowledge Is Power

Because breast cancer often develops in fibroglandular tissue, individuals with a higher proportion of this tissue face an increased risk of the disease. Unfortunately, dense breast tissue can also mask tumors on mammograms, potentially delaying diagnosis and treatment.

However, dense breast tissue is only one of several risk factors for breast cancer. Other contributing factors may include:

• Lifestyle choices, such as alcohol consumption, being overweight, or a lack of regular physical activity
• Not having children or not breastfeeding
• Use of hormone replacement therapy to manage menopausal symptoms
• Inherited gene mutations, such as BRCA1 or BRCA2
• A family history of breast cancer

If mammogram results indicate dense breast tissue, it is important to consult a healthcare provider to discuss next steps. Working collaboratively, individuals and their providers can take proactive measures to monitor breast health, manage risk factors, and promote early detection for the best possible outcomes.

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Kayleigh Taylor, MD, diagnostic radiology