Supplemental Screening for Dense Breasts

Since September 10, 2024, the FDA requires every mammography facility in the US to tell you whether your breasts are dense, using standardised wording. If yours are dense, the letter says that dense tissue "makes it harder to find breast cancer on a mammogram and also raises the risk of developing breast cancer," and that in some people other imaging tests may help.

That letter is a notification, not a recommendation. It stops short of telling you what to do, because the evidence stops short too.

Radiologists sort breast tissue into four categories:

• Almost entirely fatty — about 10% of women
• Scattered areas of fibroglandular density — about 40%
• Heterogeneously dense — about 40%
• Extremely dense — about 10%

The last two count as "dense," so roughly half of women who have mammograms are told they have dense breasts. It is not a disease, not something you caused, and not something you can change. Density tends to decrease with age.

1. It masks. Dense tissue and tumours both appear white on a mammogram, so cancer is more likely to be missed.
2. It raises risk, modestly, and independently of the masking effect.

Worth holding alongside those: after accounting for other health factors, people diagnosed with breast cancer who have dense breasts are no more likely to die of breast cancer than those with fatty breasts.

The clearest evidence is the Dutch DENSE trial, which randomised 40,373 women aged 50 to 75 with extremely dense breasts and a normal mammogram.

• Interval cancers — those found between screening rounds — fell from 5.0 to 2.5 per 1,000 screenings in the group invited to MRI.
• Among women who actually had the MRI, it detected 16.5 cancers per 1,000 screenings.
• The false-positive rate was 79.8 per 1,000 MRIs. Of the biopsies MRI prompted, about 26% found cancer — meaning roughly three in four did not.
• Only 59% of the women invited to have an MRI went ahead with it.

That is the trade in one paragraph: more cancers found earlier, and substantially more false alarms, extra appointments and benign biopsies. The trial did not show — and was not designed to show — that supplemental MRI lowers breast cancer deaths.

Ultrasound finds fewer additional cancers than MRI and still adds false positives. Contrast-enhanced mammography is newer, with less long-term data.

• The USPSTF (2024) concluded the evidence is insufficient to recommend for or against supplemental ultrasound or MRI, regardless of density.
• The American Cancer Society does not recommend MRI when density is the only risk factor.
• The ACR and NCCN are more permissive, suggesting supplemental imaging be considered, generally preferring MRI or another contrast-based test over ultrasound where available.
• The NCI states plainly that there is not yet enough evidence to recommend for or against additional imaging.

None of these groups is being careless. They are weighing the same trial results against different thresholds for what counts as proven benefit — and only one of them, the USPSTF, requires evidence of a change in health outcomes.

Whether supplemental imaging makes sense for you depends on more than density: family history, known inherited variants, chest radiation before age 30, prior biopsies showing atypia, and your overall estimated lifetime risk. If a formal risk assessment puts your lifetime risk at roughly 20% or higher, annual MRI alongside mammography is recommended by ACS — and that rests on risk, not on density alone.

One last practical point: supplemental screening for density alone is not always covered, and coverage varies by state. Ask what it will cost before you schedule.

• FDA — Final Rule to Amend the Mammography Quality Standards Act (breast density notification)
• Supplemental MRI Screening for Women with Extremely Dense Breast Tissue (DENSE trial, NEJM 2019)
• USPSTF — Breast Cancer: Screening (2024)
• American Cancer Society — Recommendations for the Early Detection of Breast Cancer