Do increased breast cancer screenings save lives? Doctors can't agree.

Roughly half of American women over 40 have what are known as dense breasts, which make screening mammograms less effective for detecting cancer. But in April the United States Preventive Services Task Force, the group charged with making public health recommendations, stated that current evidence is insufficient to determine whether these women might benefit from additional screening with breast ultrasonography or magnetic resonance imaging (MRI). 

The reason? Too few multi-year, randomized clinical trials have been done that show these supplemental screenings save lives, according to the task force’s evidence review. The group made a similar finding in its prior recommendation in 2016. 

The reviewers “could not find any studies where they could clearly show whether supplemental MRI or supplemental ultrasound reported evidence of reduced progression to advanced cancer,” says John Wong, a professor of medicine at Tufts University Medical School and vice chair of the USPSTF, who helped craft the new breast cancer recommendations that also lowered the age most women should begin biannual screening mammograms from 50 to 40.

Dense breasts contain higher levels of fibrous and glandular tissue alongside the fat that gives breasts their size and shape. The issue of supplemental screening is important because higher density is linked to up to six times greater risk of developing breast cancer. 

When performing a mammogram on dense breast tissue, “there’s more interference,” says Kelsey Hampton, director of education at the Dallas-based nonprofit breast cancer research and advocacy foundation, Susan G. Komen. “It’s like trying to look through a glass jar full of clear water versus a glass jar full of water with ice cubes. You can still see things, but it’s more difficult to see with the same level of detail.” Still, she says, mammograms are important for these women. 

(When should you get screened for breast cancer—and how often?)

Many gynecologists routinely prescribe supplemental ultrasound or MRI along with mammography in women with a dense-breast diagnosis. The task force’s new “insufficient evidence,” or “I,” rating does not mean that no women will benefit from the supplemental screening. 

“We are urgently calling for more research on whether and how additional screening might help women with dense breasts find cancers earlier,” the final recommendation statement says.

But the rating will likely confuse physicians about whether to continue prescribing the extra test and whether some insurers might drop coverage for it, says Wendie Berg, a distinguished professor of radiology at the University of Pittsburgh who disagrees with the task force’s stance. Berg calls the task force’s I rating “stunning” because she believes current evidence is sufficient to recommend these screens. 

Amplifying the confusion: Beginning in September, the U.S. Food and Drug Administration will require all women getting a mammogram to be notified which of the four levels of density describe their breasts and to be advised that “in some people with dense tissue, other imaging tests in addition to a mammogram may help find cancers.”

Screening methods are underfunded

Initial studies on supplements to mammograms go back more than 20 years. In the intervening decades, however, too little research was done to convince the USPSTF that additional screening is worthwhile.

Yet, compared to many other medical conditions, breast cancer is a well-funded field of research. The disease received more funding globally than other cancers between 2015 and 2020, some $2.7 billion, according to a study in the Lancet. Each year, the U.S. National Cancer Institute devotes more than half a billion dollars to breast cancer research, and that amount is supplemented by funding from nonprofits like Komen, which has supported more than 550 clinical trials costing more than a billion dollars since 1982, according to the organization.

Screening research doesn’t get the bulk of these funds, the Lancet study found. The vast majority went to studying cancer biology followed by drug treatment, immunotherapy, and surgery.

In making its assessment, the task force evaluated a handful of randomized screening trials that tracked the impact of supplemental screenings.

In one, for example, Japanese researchers studied a cohort of 70,000 women with all levels of breast density and randomly assigned half of them to either a group that received screening ultrasound plus mammography or just mammography alone. In the months following one round of screening, they did not find differences in so-called interval cancers, a measure used to demonstrate a screening tool’s potential benefit. This study is ongoing, and additional rounds of screenings could yield different results. 

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Researchers in the Netherlands are also in the middle of a multi-year study. To date, they have published results from two rounds of MRI screenings that included nearly 3,500 women with extremely dense breasts whose mammograms were negative. In the second round, six extra cancers for each thousand women were detected. But the test also flagged 26 cases as potential cancers that were not. These false positives require unnecessary additional procedures with accompanying heightened anxiety. This rate was lower than the false positives in the first round, where nearly 80 occurred.

Increased detection rates by themselves do not indicate benefits for the techniques, Wong says. For example, if cancers that are flagged by supplemental screening are slow growing, they could likely be treated as effectively had they been caught at the next, regularly scheduled mammogram.  

“When you look harder, you are going to see more,” Wong says. “But is there definitive benefit, where patients can clearly live longer from that positive supplemental screen? We’re not there.” 

Some studies excluded

Not everyone agrees the evidence is inconclusive. The European Society of Breast Imaging advocates for supplemental screening with MRIs every two to four years for women with extremely dense breasts. And the American College of Radiology recommends women with higher than average breast cancer risk get screened annually with mammograms and supplemental MRIs.

Berg was disappointed the task force excluded some studies from its evaluation because they were not randomized controlled trials. One of the excluded studies involves more than a thousand women with dense breasts who have undergone two annual mammograms supplemented with an MRI. In that research, four additional cancers were detected with MRI, many of them invasive. Other studies found screening ultrasounds detect an additional two to three cancers per thousand on average, many of them invasive. 

Berg is convinced all the research taken together supports supplemental screening, and has created a website to educate patients and physicians.

Some women will receive false positive results requiring unnecessary follow up, she concedes. But in a review published in 2019 she states that careful training of technicians along with the increased use of supplementing radiologist readings with artificial intelligence can minimize this rate.

Worries about insurance

The USPSTF states that to potentially change its evaluation of additional screening in the next review cycle—likely some five years out—more studies are needed that report health outcomes such as quality of life and mortality, especially those done in settings applicable to primary care in the U.S. 

Additional years of screening from the major ongoing studies may yield the data the task force needs, Wong says. 

In the meantime, women should speak to their healthcare practitioners to understand both their breast density level and their overall breast cancer risk, Hampton says. Density can shift over time, especially as people age, change their body weight, become pregnant or breastfeed, or take menopausal hormone therapy.

(Breast cancer spreads more aggressively during sleep)

Those with dense breasts should talk to their provider about prescribing a supplemental screen along with a mammogram, Berg says. MRI is more effective than ultrasound, but the technology is not as widely available and the cost to the patient may be higher. Some people refuse MRI screens for additional reasons. They were offered free in the Dutch study, but 40 percent of women declined, citing inconvenience, concern about the needle injection inserting the contrast material, and claustrophobia while in the machine.

In the U.S., whether supplemental screens must be covered by insurance alongside mammograms varies by state law. Some observers worry insurers may use the USPSTF I rating as justification for change.

“There is a potential that by classifying these supplemental services as inconclusive, it could lead to insurance companies no longer covering these services,” Hampton says.

Berg believes catching breast cancer early is so important that women with dense breasts should not be dissuaded by the USPSTF rating. “A woman should have enough information to make her own choice and ask her doctor for the prescription if she wants it,” she says. “That's the bottom line.”