Your mammogram says dense breasts: do you need more imaging?

The wording in your letter is set by federal regulation. The FDA published its amended Mammography Quality Standards Act (MQSA) rule on March 10, 2023, and the Federal Register entry states: "This rule is effective on September 10, 2024." Under 21 CFR 900.12, the radiologist's report to your clinician must place your breasts in one of four density categories, and the plain-language summary sent to you, within 30 calendar days of the exam, must carry one of two fixed statements depending on which category you fall in.

If you are in category C or D, the regulation requires this exact paragraph: "Breast tissue can be either dense or not dense. Dense tissue makes it harder to find breast cancer on a mammogram and also raises the risk of developing breast cancer. Your breast tissue is dense. In some people with dense tissue, other imaging tests in addition to a mammogram may help find cancers. Talk to your healthcare provider about breast density, risks for breast cancer, and your individual situation." The not-dense version has the same opening and closing sentences, says "Your breast tissue is not dense," and omits the sentence about other imaging tests.

Two things follow. First, your letter collapses four categories into two, so "dense" covers both heterogeneously and extremely dense tissue, which behave differently in the research below. The category letter is in the report your clinician receives, and it is worth asking for. Second, the rule deliberately stops at notification. In its response to public comments, the FDA wrote that "this final rule does not specify any particular supplemental imaging modality or other particular clinical management of patients with dense breasts," and that it "has not indicated any particular additional steps in a patient's care based only on the mammogram, as individual situations and risk factors vary." On cost: "Insurance coverage and reimbursement are outside the scope of these regulations."

A study from the Breast Cancer Surveillance Consortium (BCSC), using 1,518,599 mammograms from 2007 to 2010, estimated that 43.3% of US women aged 40 to 74 had heterogeneously or extremely dense breasts, about 27.6 million women, and that the proportion falls with age and with body mass index (Sprague et al., Journal of the National Cancer Institute, 2014; abstract). The USPSTF puts it as "Nearly half of all women have dense breasts." In the FDA's words, "Dense breasts have a higher proportion of fibroglandular tissue compared to fatty tissue."

It matters in two ways, and the USPSTF names both: "Dense breasts are associated with both reduced sensitivity and specificity of mammography and with an increased risk of breast cancer." The FDA's required wording does not include the USPSTF's next sentence: "increased breast density itself is not associated with higher breast cancer mortality among women diagnosed with breast cancer, after adjustment for stage, treatment, method of detection, and other risk factors, according to data from the BCSC." The study behind it followed 9,232 women with invasive breast cancer and found a hazard ratio for breast cancer death of 0.92 (95% CI, 0.71 to 1.19) for the densest category against the scattered-density reference (Gierach et al., 2012; abstract).

An I statement is not a recommendation against. The USPSTF tells clinicians to "use their clinical judgment" here, and its Q and A adds that "these women should talk to their clinicians about their options for follow-up testing." Its stated reason for not going further is that "No studies of supplemental screening reported on health outcomes or on the incidence of and progression to advanced breast cancer over more than 1 round of screening." ACR's abstract does not set out its reasoning, and we read only the abstract.

The strongest evidence is one randomized trial of MRI. DENSE, run in the Netherlands and published in the New England Journal of Medicine in 2019, invited women aged 50 to 75 with extremely dense breasts (category D only) and a normal screening mammogram to have an MRI, and compared them with women who had mammography alone. We read the abstract.

Interval cancers, the ones diagnosed between screens, halved in the group offered MRI. The cost was about 80 false positives per 1,000 MRI screens, and roughly three of every four biopsies prompted by MRI found no cancer (our arithmetic from the 26.3% figure). The trial measured interval cancers, not deaths, and it enrolled only the extremely dense group, so it says nothing direct about heterogeneously dense breasts. A US insurance-claims study summarized by the USPSTF found women who had an MRI were more likely over the next 6 months to have additional "cascade events" related to findings outside the breast, 19.6 more per 100 women screened, "mostly additional health care visits."

The ultrasound evidence is thinner. The USPSTF reports that a randomized trial adding ultrasound in women aged 40 to 49, not selected for dense breasts, found no statistically significant difference in interval cancers (relative risk 0.58, 95% CI 0.31 to 1.08), and neither did a BCSC comparison (adjusted relative risk 0.67, 95% CI 0.33 to 1.37), while in that BCSC analysis "the rates of referral to biopsy and false-positive biopsy recommendations were twice as high and short interval follow-up was 3 times higher for the group screened with ultrasonography."

Your letter cannot tell you your overall risk, and in the research that is what separates high interval cancer rates from low ones. A 2015 BCSC cohort of 365,426 women aged 40 to 74 (Kerlikowske et al., Annals of Internal Medicine; abstract) combined density with each woman's 5-year breast cancer risk. High interval cancer rates, more than 1 per 1,000 mammograms, appeared only in women with a 5-year risk of 1.67% or more and extremely dense breasts, or 2.50% or more and heterogeneously dense breasts: 24% of the women with dense breasts. About half of women in each dense category had low to average 5-year risk. The authors' conclusion: "Breast density should not be the sole criterion for deciding whether supplemental imaging is justified because not all women with dense breasts have high interval cancer rates."

ACR and ACS both build on a risk estimate. ACR's 2023 recommendations say "All women should undergo risk assessment by age 25." ACS sets a separate high-risk track, with a breast MRI and a mammogram every year, typically from 30, for women with, among other criteria, a lifetime risk of about 20% to 25% or more on family-history-based tools, a BRCA1 or BRCA2 variant, or chest radiation before 30. ACS also "recommends against MRI screening for women whose lifetime risk of breast cancer is less than 15%." On our reading, for ACS the risk estimate comes before the density label.

The USPSTF treats digital breast tomosynthesis (DBT), sold as 3D mammography, as a primary screening test rather than a supplemental one, stating that "both digital mammography and DBT are effective primary screening modalities." Its review found that the difference in false-positive results between DBT and standard digital mammography "was largest for women with nondense breasts and was not significantly different among women with extremely dense breasts." ACS writes that "several studies have shown it can be helpful in women with dense breasts," and that 3D mammograms often cost more, an added cost that "might not be covered by insurance."

Ask for your density category, C or D, and for a formal breast cancer risk estimate; together they place you in the research above more precisely than the word "dense" alone. If supplemental imaging is offered, ask which test, what your plan will pay, and what happens after an abnormal result, since both the MRI and ultrasound studies above found more false positives or biopsies. Our breast cancer screening page carries each body's start age and interval, quoted and dated, and your personal screening plan lists everything that applies at your age and sex. If you are 75 or older, mammograms after 75 covers the separate question of whether to keep screening at all. The trade at the center of this decision, finding more against more false alarms, is the same one we lay out for colorectal screening in Cologuard versus colonoscopy.