Up to 19% of women are being incorrectly determined as having dense or non-dense breasts due to inconsistency in measuring breast density. Moreover, supplemental breast cancer screenings for women with dense breasts are greatly increasing the chance of both true and false positive results, says research published in the Annals of Internal Medicine.

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A mammogram highlights dense and non-dense breast tissue.

Breast tissue is composed of milk glands, milk ducts and supportive tissue, which is the dense breast tissue, and fatty tissue, which is non-dense breast tissue.

Non-dense breast tissue appears dark and transparent on a mammogram, whereas dense breast tissue appears as a solid white area, making it harder to see through and potentially increasing the risk of missing a breast cancer diagnosis. Higher breast density also modestly increases a woman’s risk of developing breast cancer.

Women with more dense tissue than fatty tissue are considered to have dense breasts.

The Breast Imaging Reporting and Data System (BI-RADS) is used to categorize breast density.

Breasts are defined as (a) mostly fatty, (b) scattered density, (c) consistent density or (d) extremely dense. Women classified in categories (c) and (d) may be offered supplemental screening with ultrasound or magnetic resonance imaging (MRI).

It is becoming increasingly common for US states to mandate that breast density information be reported to patients.

However, the current study shows this approach may not be supported by strong scientific evidence, as it appears to be difficult to assess accurately whether a woman has dense breasts to begin with.

Researchers from the University of California-Davis systematically reviewed 24 studies to examine evidence on the consistency of breast density category assignment and on supplemental screening beyond standard mammography for women with dense breasts.

One study showed that different radiologists assigned the same BI-RADS category to an individual woman only 82% of the time. Other data showed that as many as 22% of women were reclassified from dense to non-dense, or vice versa, on consecutive mammograms.

Additional diagnostic tests for women thought to have dense breasts find some additional breast cancers; however, no studies have examined women’s long-term health outcomes after supplemental screening beyond diagnosis.

Joy Melnikow, director of the Center for Healthcare Policy and Research at UC Davis and first author on the paper, would like to see long-term, rigorous research to achieve better standardization of breast density classification and determine whether supplemental screenings provide actual health benefits for women.

A statement made by the US Preventive Services Task Force (USPSTF) in the same publication says there is insufficient evidence to assess the balance of benefits and harms for supplemental screening of women with dense breasts with ultrasound, MRI or other modalities.

Melnikow says:

It is important to be clear who actually has dense breasts. Also, when patients are told their breasts are either dense or not dense, they need to have confidence in that assessment. It is important that policies come from the evidence. It is also important that women not overreact to information about their breast density.”

Melnikow suggests that “the policy may be out in front of the science,” and questions whether using mammography and ultrasound to find more breast cancers is really beneficial to women’s health.

In a linked editorial, Dr. Wendie A. Berg, of Magee-Womens Hospital of the University of Pittsburgh, PA, says that women with dense breasts who are interested in supplemental screening should be offered the chance, but adds that guidance to help them to make the right choice is “sorely needed.”

Medical News Today recently reported that a high sugar intake may increase the risk of breast cancer.