If you're a woman who gets screening mammograms, you may have received a letter telling you that your scan was clear, but that you have dense breasts, a risk factor for breast cancer. About half of U.S. states require providers to notify women if they fall into that category.
But what you may not know is that gauging breast density isn't a clear-cut process. Researchers reporting in Annals of Internal Medicine Monday found that density assessments varied widely from one radiologist to another. That means you shouldn't let one finding freak you out too much, nor should you assume something's wrong if your reported density changes from year to year.
"Women and providers should keep in mind that density is a subjective measure," says Brian Sprague, a cancer epidemiologist at the University of Vermont and an author of the study. And, he says, breast density is only one factor contributing to a woman's individual risk of getting breast cancer.
About 40 percent of women between 40 and 74 years old have dense breasts — meaning they have more breast tissue (that is, ducts and glands) and connective tissue and less fatty tissue than women whose breasts aren't dense. You can't know your status by how the breasts feel; it only shows up on a mammogram.
Dense breasts make it harder for radiologists to detect possible abnormalities on a mammogram, and the presence of the tissue itself is an independent risk factor for breast cancer.
The researchers looked at 216,783 mammograms from more than 145,000 women, interpreted by 83 radiologists in Pennsylvania, Vermont, New Hampshire and Massachusetts. The average proportion of mammograms that fell into the "extremely dense" or "heterogeneously dense" categories was 38.7 percent. But the proportion of mammograms assigned to those two categories by individual radiologists ranged from 6.3 percent to 84.5 percent.
Even when adjusting for each patient's age, race and body mass index — since, after all, the patient population in Philadelphia isn't same as in rural Vermont — the variation continued, the authors say.
And among women who had consecutive mammograms read by different radiologists, 17.2 percent got different assessments of whether they fell into the dense or nondense category.
The findings aren't too surprising, says Dr. Priscilla Slanetz, a radiologist at Beth Israel Deaconess Medical Center. "There's agreement usually in the extremes, but a lot of variation in the middle," she says.
The guidelines for assessing density have also changed since the study was conducted, Slanetz points out, though it's not yet clear how that will affect the percentage of women assessed as having dense breasts.
At a policy level, the researchers say, the results mean that authors of state legislation requiring that women be notified of breast density — and in some cases, offered extra screening using other methods — need to be aware that this variation exists. If all women classified as having dense breasts are referred for an ultrasound based on that factor alone (as they are in some states), that could make for a lot of unnecessary tests and false positive results without an offsetting benefit.
Slanetz's advice for women is to use the density report as a jumping-off point for a broader discussion about their individual breast cancer risk. Authors of a large study published last year said that density alone shouldn't be the only criterion for getting extra screening. Nor should women whose breasts aren't dense assume that they have a low risk of breast cancer.
That personal discussion with a doctor or other health care provider should cover risk factors such as personal history of breast abnormalities and family history of breast cancer, as well as density, Slanetz says.
She also recommends that women with dense breasts seek out digital mammography, which improves detection of cancers.
Ultrasound, digital breast tomosynthesis and MRI have all been suggested as additional screening options for women with dense breasts. But the U.S. Preventive Services Task Force says there's not yet enough evidence to know whether they should be used for screening.
Katherine Hobson is a freelance health and science writer based in Brooklyn, N.Y. She's on Twitter: @katherinehobson