The Merriam-Webster dictionary provides three essential meanings for the word “density”: having parts that are close together; being difficult to see through; and being difficult to understand. In the case of “breast density,” it is the second meaning that applies. In 2015, two breast cancer survivors – JoAnn Pushkin and Wendie Berg – joined forces to launch an educational website, DenseBreast-info.org (DB-I), to provide information on breast density to patients and healthcare professionals. For the past seven years, DB-I has provided valuable insights for understanding breast density and its impact on early detection of breast cancer.
Breasts are made up of three tissue types: fat; lobules and glands (for milk production); and fibrous connective tissue. The relative proportion of these tissues determines the breast density which may be measured on a mammogram using the BI-RADS scale (seen below left): (A) almost entirely fatty; (B) scattered fibro-glandular density; (C) heterogeneously dense; and (D) extremely dense. These four categories are influenced by a range of factors, including age, hormone therapy, lactation, weight changes and medication.
Over 40% of women have dense breasts – categories C and D – and the connective tissue, which appears white on the mammogram, can mask a malignant tumour. This can lead to a false negative diagnosis in up to 40% of women, with potentially devastating consequences. Dense breast tissue not only reduces the effectiveness of mammography it is also a significant risk factor for developing breast cancer: the denser the breast, the higher the risk. In fact, women with extremely dense breasts have a 4-fold greater risk than women with fatty breasts.
As part of its educational outreach, DB-I has been running a series of quizzes under the headline, “In the Know with DenseBreast-info.” There have so far been four multiple-choice questions. How is a woman’s breast density determined? Breast density increases breast cancer risk: true or false? For which density categories does digital breast tomosynthesis (DBT) outperform full-field digital mammography (FFDM)? What breast screening modalities are appropriate for women with dense breasts? Questions are followed by detailed explanations.
The correct answer to the question at right is D, which means that, somewhat counter-intuitively, breast density cannot be determined by the way a breast feels. Another surprising answer is that while DBT identifies more cancers than FFDM for the first three BI-RADS categories, this is not the case for extremely dense breasts. This strongly suggests that another imaging modality such as automated breast ultrasound (ABUS) should be offered to women to minimise false negative findings. Clearly, understanding breast density is important for both patients and healthcare providers.
The milk ducts are another small component of “dense” tissue (not mentioned).
The “density”’ of breast tissue is determined by its x-ray attenuation. Fat in the breast (actually its major component in most women) is like “window glass” on a mammogram. X-rays penetrate fat easily. The other tissues have higher water content and attenuate x-rays more than fat and thus cast shadows on the mammogram. Breast cancers, which are almost completely cellular (usually with a minimum of fat, if any) are visible on x-rays with similar attenuation as the normal, other water density structures. Radiologists dating back to John Wolfe, who promoted Xeroradiography and was the first to divide breasts into several types of based on their “density”, was the first to suggest that breast density was a risk for developing breast cancer, although his estimates were grossly exaggerated. In fact, studies linking density to risk are highly suspect. They have all tried to gauge the percent of the breast that is dense tissue compared to the amount of fat. All such estimates suffer from the fact that the breast has no definable boundaries – where does it end? Without being able to accurately measure the total volume of the breast, it is impossible to measure the percent of the volume that is dense (percent volume = volume of dense tissue / volume of the breast ).
Claims of increased risk due to dense tissue are greatly exaggerated. In our data, and I would bet your own, the same percentage of women in the population that have a particular pattern is similar to the percent of women diagnosed who have each pattern. The problem is that dense tissues can hide cancers and the delay in their detection is likely the source of claims of increased risk (cancers are found earlier in fatty breasts). It is paradoxical to suggest that dense breasts are at higher risk when virtually all women (90%) in their twenties and early thirties have “dense” breasts yet they have the lowest risk of breast cancer. Perhaps it is the persistence of dense breast tissue into the postmenopausal years that is a risk (? hormones ?), but this is not known.
Regardless, dense tissues can hide cancers on mammograms. Some are visible on ultrasound, and even more are visible on MRI. Until we can screen effectively and efficiently using ultrasound and MRI, mammogrpahy remains the mainstay of early detection.
Many thanks for this excellent — and insightful — addition to my blog, Dan. Your point about increasing breast density and the risk for breast cancer is well made. Your last sentence is particularly pertinent to those of us involved in developing new screening tools for early detection of breast cancer.