In a long-awaited ruling, the Food and Drug Administration (FDA) recommended Thursday that all mammography centers inform women if they have dense breasts that could increase the risk of breast cancer.

The density of breast tissue, whether it contains primarily fatty or glandular tissue, varies from woman to woman and has nothing to do with the size, shape, or feel of a woman’s breasts. The only way for a woman to know if she has dense breasts, and if so, how dense they are, is to have a mammogram; she can’t tell by looking or feeling her breasts.

Dense breast tissue can hide cancer risks because it can be more difficult to detect small tumors with a mammogram.

Breast imaging divides density into four levels, labeled A, B, C, and D, ranging from almost completely fatty breasts to extremely dense breasts. Only 10 percent of women in level D, with extremely dense breasts, are at increased risk. Those in level C have an average risk of cancer, based on density alone.

The updated regulations require patients to simply be told whether their breasts are dense or not dense; levels A and B would be “non-dense”, while C and D are “dense”.

FDA will evaluate facilities to determine whether to issue a certificate verifying compliance with the agency’s new requirements. If a woman doesn’t see a center’s certificate, she can request it or check the FDA’s website, says Dr. Said Hilary Marston, the agency’s chief medical officer.

JoAnn Pushkin, CEO of DenseBreast-info, Inc., an educational group, praised the agency’s update. Although 38 states have laws requiring mammography facilities to tell women if their breasts are dense, those rules vary in the amount of information they require women to receive. Because the new regulations standardize what must be disclosed to her, she said, “it will lead to earlier detection of breast cancer.”

But the picture with dense breasts and screening isn’t so straightforward, said Dr. Christoph Lee, a breast imaging specialist at the Fred Hutchinson Cancer Center and director of the Northwest Cancer Detection and Outcomes Research Company. from the University of Washington. He fears that the information women get about the density of their breasts could be misleading.

Only a small minority of women with dense breasts are at increased cancer risk based on breast density alone, says Dr. Lee, adding that “of all the clinical risk factors, breast density is only moderate.” Of much greater importance, he said, are risk factors such as family history and prior breast cancer.

dr. The FDA’s Marston said many doctors already get more detailed information because many states require reporting. “Doctors will have to decide on the situation of the patients in terms of risk factors,” he said.

“The big question is: What do women do with the information?” dr. Lee asked. If a woman is told that her breasts are “dense”, what does that mean? Many women have heard, repeatedly, that if they have “dense” breasts, they need more frequent screening or additional screening with ultrasound or MRI. If they get a report that their breasts are “dense” and their doctor says they just need regular mammograms. Will you follow that guide?

The FDA’s hope is that the information, dense or not, will lead to a formal evaluation by a physician who can actually advise women if they are at higher overall risk.

But, Dr. Lee cautioned that an initial finding of breast density should not reflexively trigger further testing.

“It can be confusing for women to know what the next step should be,” she added. “There is some evidence from Europe that women in the top 10 percent of density might benefit from a supplemental MRI.” But, she added, there isn’t much evidence that women outside of the extremely dense category would benefit from supplemental screening.

And even if women want such an evaluation, insurers often won’t cover it if a woman’s lifetime risk, according to standard risk calculators used by doctors, is less than 20 percent.

The new regulations, Dr. Lee said, “are a step toward informing women, but it’s not clear where they will lead.”