Approximately 40-50% of women over the age of 40 are considered to have dense breasts. The breast is normally composed of fat and glandular tissue – the higher the proportion of glandular tissue, the denser the breast. Breasts usually become less dense with age due to a decline in hormone levels. Dense breast tissue appears “white” on a mammogram, and the challenge is that cancers also will often appear as a white mass on mammogram. “Lumpy” breast tissue is not the same as dense breast tissue – density is determined by the appearance on a mammogram, not by feel. The radiologist (MD specializing in reading imaging studies) will estimate the approximate percentage of dense tissue versus fat on the mammogram. Density is then graded (either 1-4 or A-D). Density grades of 3 or C (heterogeneously dense) and 4 or D (extremely dense) are considered “dense.” Density grading currently is subjective (depends on the eye of the radiologist) and different radiologists may score the same mammogram differently.
In addition to possibly “hiding” a cancer, density is one of many factors associated with breast cancer risk. However, not all women with dense breasts are at increased risk for breast cancer, and density may not contribute equally to level of risk in all women.
While mammograms are the “gold standard” for breast cancer screening and detection, approximately 10-20% of breast cancers are missed by mammograms, and that percentage can approach 40-50% in women with dense breasts. Over the past several years, there has been an increased awareness of the significance of dense breasts. Mammogram reports contain a notation of breast density, but the “layperson letter” that the patient receives did not always contain this information. Several states, including California, have passed legislation requiring that patients be informed of their breast density.
Tomosynthesis, also known as 3D mammography, can improve the rate of breast cancer detection in women with dense breasts (picking up approximately 6-7 cancers per 1000 exams versus 3-5 cancers per 1000 exams with 2D mammography). There are no long-term studies to show if outcomes are improved, and there are still a few insurances that do not cover tomosynthesis. While we may be able to detect additional cancers with these newer techniques, concern has been raised about overdiagnosis and overtreatment – finding lesions that will never be of any significance to the patient. However, studies have shown that the use of 3D mammography reduces the likelihood of a “callback” – when additional imaging is recommended due to inconclusive results because of dense tissue.
Other breast imaging modalities include MRI and ultrasound. MRI is generally reserved for those found by risk assessment to have an elevated risk of breast cancer. Again, while dense breasts are a cancer risk factor, not all women with dense breasts are at increased cancer risk. MRI is associated with an approximately 10-20% false positive (false alarm) rate and there are possible concerns related to the need for gadolinium, a heavy metal intravenous contrast.
The use of supplemental screening ultrasound has become more common. Supplemental ultrasound can find 0-5 additional cancers (versus mammogram alone) per 1000 studies. However, it is associated with an approximately 20-30% false positive rate, and an increased rate of benign (non-cancerous) biopsies and recommendations for follow up study. Because ultrasound was never designed as a full-breast screening tool, insurance may not cover the exam and recommended follow up. There are no studies demonstrating that cancer outcomes are improved using supplemental screening ultrasound.
It is important to realize that there is no perfect imaging technique for dense breasts – all available modalities have the potential to miss cancer. My recommendation is that women speak to their doctors and ask questions. Ask if your breast tissue is dense, how density impacts your breast cancer risk, and if additional testing is recommended for you.
Dr. Attai was one of the physicians invited to testify before the California Legislature in support of SB 1538, which was eventually signed by Governor Brown, requiring that imaging facilities in California inform women if they have dense breasts on mammography.
Updated January 31, 2022