Breast Density

About the Author: Dr. Brian E. Juell, MD. FACS has specialized in the treatment of patients with breast cancers for over 20 years. He is the Medical Director Renown Breast Center and is the only surgeon in Northern Nevada recognized by the American College of Breast Surgeons as Certified in Breast Ultrasound. 

Breast Density is now a standard reporting measure on mammography reports. In Nevada and other states breast density must be reported to patients on results of screening mammograms. The density of the breast refers to the appearance of the breast tissue on the captured mammographic image. Fat content of the breast allows the radiation beam to pass without deflection through the breast, making the image appear darker or more exposed. Dense glandular tissue scatters the radiation beam passing through the breast. This deflection of the beam results in less exposed images which appear lighter and more opaque on the mammogram. Increasing density on mammogram decreases the detection rate of breast cancers and increases the potential false negative rate of this valuable screening exam.

There are four categories of breast density. These categories are referred to as BI-RADS assessments.  BI-RADS stands for the Breast Imaging Reporting and Data System. These categories are:

BI-RADS Category                            Description                                         % of Population

1                                              almost entirely fat                                           10%

2                                              scattered glandular density                         40%

3                                              more gland, less fat                                        40%

4                                              extremely dense                                             10%

Breast density is related to breast size and overall body weight. In general the smaller the breast, the more dense the breast. Breast density also tends to decrease with maturity as the gland is less stimulated by hormonal influences particularly after menopause. About 80% of women less than 40 years of age have dense breasts. This is one reason that screening mammograms are not recommended for most women until age 40.

Breast density is an independent risk factor for Breast cancer development. The relative life time risk for BI-RADS category 3 density compared to average breast density is 1.2 x average. For extremely dense BI-RADS category 4 patients the relative lifetime risk is 2.1 x average. Of course other risk factors are important including:

Age (Breast Cancer incidence increases with age)


Family History of Breast and other Cancers (possible genetic predisposition)

Previous Breast Biopsy with atypical pathology including:

Atypical Ductal Hyperplasia (ADH)

Lobular Neoplasia (ALH, LCIS)

Previous Chest Wall Radiation Therapy

Tobacco and Alcohol Use

Hormone Replacement Therapy

Prolonged Menstrual History (early menarche and late menopause)

Parity and Breast Feeding History

Younger first pregnancy, more pregnancies and breast feeding decrease risk

Patients should consult with their Physicians about assessing their risk of breast cancer and the management of those risks. Patient’s individual tolerance of risk will enter into the discussion.  Patients with dense breasts may have additional screening options beyond mammograms. Additional screening imaging in most cases may not be covered by insurance. Patients with a lifetime risk of breast cancer exceeding 20% are eligible for MRI screening covered by insurance. Breast density is not a covered indication for MRI screening.

The U.S. Preventive Task Force position is that there is currently insufficient evidence to recommend additional screening based on breast density alone. There are no prospective randomized trials studying patients with breast density as the only risk factor. There is no increased risk of dying from breast cancer when risk is adjusted for density.

Standard 2 view (2D) mammography has a cancer detection rate (sensitivity) of 89% for fatty breasts and 63% for extremely dense breasts. Mammography alone detects about 50% of cancers with a cancer yield of 5/1000 patients screened. Digital mammography is better than film screen mammography. Interval cancer rates are therefore around 50% as well.  Interval cancers may be up to 18 times more common in dense breasts. .  Interval cancers are those missed or masked on initial screenings and found on subsequent imaging. Interval cancers have up to a 3 fold increase risk of mortality compared to screen detected cancers.

Tomosynthesis or 3D mammography (Currently available at Reno Diagnostic Center and scheduled to arrive at Renown in 2017) has a cancer yield of 4-8/1000 patients or an increase of 0.5-2.8/1000 compared to 2D mammography. There is a small increase in the amount of radiation exposure. Studies have also demonstrated a decrease in call back rates for tomosynthesis.

Whole breast ultrasound, either physician hand held or automated (3D, Sonocine) has a cancer yield for patients with negative mammograms of 2.7-4.6/1000 patients studied. There is no radiation exposure with ultrasound. In the J-START trial, a prospective randomized trial in Japan involving 73,000 women reported in 2016 comparing mammogram vs. mammogram +ultrasound there  was an increase cancer yield of 57% and reduction of interval cancer detection rates by 50% favoring the addition of ultrasound screening. Automated ultrasound is available at Renown.

MRI screening is a radiation free modality relying on the detection biologic activity of cancer related to increased blood supply and function of tumors. It is highly sensitive but does have an increased false positive rate. Compared to other screening exams it is expensive. There have been 7 MRI screening trials with a cancer yield of 22-36/1000 patients screened and an interval cancer detection rate of 0-9.8%. MRI is available at Renown.

Molecular Breast Imaging (MBI) is a nuclear medicine screening test being studied. Patients are injected with a radioactive tracer then scanned in a special detector. Cancer yield is 10/1000 patients screened with MBI alone and 11.1/1000 with mammography + MBI.  MBI has a lower cost and lower false positive rate than MRI with a radiation dose similar to tomosynthesis 3D mammography. Unfortunately, MBI is not currently available in Northern Nevada.

There are blood tests being developed for screening patients for breast cancer. Patients with normal mammograms and a positive blood test are being studied with MRI. This research is exciting to be sure.

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