Having dense breasts is a double-edged sword when it comes to screening for breast cancer: dense breast tissue may increase the risk for breast cancer as well as complicate the detection of tumors with screening mammogram.

Whereas American guidelines do not yet recommend supplemental screening, doctors may choose to follow up negative mammograms with MRI in women with dense breasts. Until now, very little data has supported this practice.

Now, a randomized controlled trial has provided the first high-quality evidence that using MRI in addition to screening mammography may improve early detection of breast cancer in women with extremely dense breasts.

Results were published online today in the New England Journal of Medicine.

“We found that supplemental screening with MRI in women with extremely dense breast tissue resulted in the diagnosis of significantly fewer interval cancers than the use of mammography alone,” write lead author Marije F. Bakker, PhD, University Medical Center Utrecht, the Netherlands, and colleagues with the DENSE Trial Study Group.

The study showed that using MRI roughly cut in half the number of interval cancers compared with screening mammogram alone.

Interval cancers are those found during the time period between screening tests — in this case, breast tumors found during the regular 2-year interval after a negative screening mammogram and before the next regularly scheduled screening mammogram. A reduction in interval cancers may indicate that these cancers have been detected earlier, before they could become symptomatic and cause disease or death.

Results from the study also suggested that using supplemental MRI detected more slow-growing, less aggressive cancers. Many of these tumors were also early stage, well differentiated tumors that were hormone receptor-positive, and potentially more treatable than later-stage tumors.

Yet not all tumors that are detected early actually progress to cause symptoms or death. So the question remains whether detecting breast cancer earlier using MRI translates into improved survival for women with dense breasts.

“It is unclear how many of the cancers detected in our trial were life-threatening and what fraction, if any, represents overdiagnosis,” the authors write.

Moreover, supplemental MRI had a false positive rate of 8%. And almost 74% of women who had MRI and went on to breast biopsy did not actually have breast cancer.

“[If we use supplemental MRI], will we be putting these women at increased risk of procedures without contributing to their eventual survival?” asked Dan Longo, MD, in an accompanying editorial. Longo, who was not involved with the study, is a professor of medicine at Harvard Medical School in Boston, Massachusetts, and deputy editor of the New England Journal of Medicine.