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The Breast Cancers Mammograms Are Likely to Miss

But more testing may be unnecessary and costly, experts say

For many women, detecting cancer on a mammogram might be as difficult as finding that proverbial polar bear in a snowstorm.

That’s because they have dense breasts, which contain more fibrous or glandular tissue than fat. (Firm breasts are not necessarily dense breasts, which is something that can be determined only by mammography, not by touch.)

Fibrous and glandular tissue show up white on a mammogram, same as cancer, while fat is pretty much transparent. In some women, breast density declines after menopause, but in others it does not. Not only is mammography more likely to miss cancers in women with dense breasts, but density itself is a risk factor for breast cancer.

A recent study of more than 1.5 million mammograms found that about 43 percent of women ages 40 to 74 — or an estimated 27.6 million in the U.S. — have dense breasts.

New Laws Require Letters

So at first glance, at least, state laws requiring that women be notified if their mammograms show they have dense breasts seem to make a lot of sense. Since 2009, 24 states have implemented breast density notification laws, while eight more have bills under consideration, according to DenseBreast-Info.org. In addition, legislation has been introduced in both houses of Congress that would require the inclusion of breast density information in screening mammogram reports for physicians and patients.

I’m always for people...being informed, but you can’t tie it to any recommendation, because we don’t know what to do yet.

— Karla Kerlikowske, M.D., UC San Francisco

The problem, though, as some recent research has pointed out, is figuring out what to do with that information.

“I’m always for people knowing and being informed, but you can’t tie it to any recommendation, because we don’t know what to do yet,” says internist Dr. Karla Kerlikowske, a professor of medicine and of epidemiology and biostatistics at the University of California, San Francisco. “Very few states say you should do an ultrasound, you should do an MRI.”

Kerlikowske leads the San Francisco Mammography Registry, part of the National Cancer Institute-supported Breast Cancer Surveillance Consortium (BCSC), and serves as co-leader of a project studying the biological basis of breast density and breast cancer risk.

‘Not Enough Evidence’ to Justify MRIs

New breast cancer screening guidelines from the American Cancer Society, published Oct. 20, echo Kerlikowske’s comment about the lack of information on how best to screen women with dense breasts. “There’s not enough evidence to make a recommendation for or against yearly MRI screening for women who have a moderately increased risk of breast cancer,” such as those with dense breasts, the guidelines say.

The first draft of new screening recommendations from the U.S. Preventive Services Task Force, posted in April, contains a similar observation. “The evidence on how additional screening with an ultrasound, MRI, or 3D mammography may or may not help women with dense breasts is unclear.” For that reason, the task force, an independent panel of experts in prevention and evidence-based medicine, said it could not make a recommendation for or against additional screening in women with dense breasts.

Because no guidelines recommend it, insurance coverage for supplemental screening of women with dense breasts is spotty, except in a few states that mandate it, Kerlikowske says.

Not All Have Higher Risk

Breast density should not be the only criterion for deciding whether additional screening is justified, Kerlikowske and her co-authors concluded in a study published in May in the Annals of Internal Medicine. The study, which involved 365,426 women aged 40 to 74, found that not all women with dense breasts had an above-average risk of being diagnosed with invasive breast cancer within a year of a normal mammogram.

And yet, the notification laws are “drawing all this attention to breast density,” says Brian Sprague, one of Kerlikowske’s co-authors and an assistant professor in the department of surgery at the University of Vermont College of Medicine. “That’s kind of one little piece of the puzzle.” Sprague leads the Vermont Breast Cancer Surveillance System, part of the BCSC.

Sprague and Kerlikowske also collaborated on a study about the benefits, harms and cost-effectiveness of following up mammograms with ultrasounds when screening women with dense breasts. Using statistical modeling, the researchers found that adding ultrasound to mammography when screening women with dense breasts “would result in limited health gains and substantially increase costs,” they wrote in December 2014 in the Annals of Internal Medicine.

Generally, breast density decreases about 1 percent a year over five years around menopause, Kerlikowske says. Some women have a bigger decrease, while others don’t change at all. “We see people who are in their 70s who have extremely dense breasts,” notes Kerlikowske.

Factors that Increase Density

As much as 60 percent of the reason women have dense breasts or not is heredity, Kerlikowske says. In addition, density can increase when a woman takes estrogen plus progestin hormone therapy, but it goes back down once she stops, she says. On the other hand, density can decrease when women take tamoxifen, a drug prescribed to high-risk women to prevent breast cancer.

If you’re curious about your risk of breast cancer in the next five or 10 years, you could try using the free BCSC Risk Calculator online. It is designed to estimate risk for women 35 to 74 who’ve never had breast cancer, a mastectomy or breast enlargement. Breast density is only one of five factors considered in the calculation. The others are: age, race or ethnicity, a history of breast cancer in a first-degree relative (parent, sibling or child) and whether you’ve ever had a breast biopsy.

The calculator was designed for use by health professionals. “If you are not a health professional,” it advises, “you are encouraged to discuss the results and your personal risk of breast cancer with your health care provider.”

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