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(CNN) —  

Getting an MRI in between regular mammogram checkups could be an effective way for women with extremely dense breast tissue to learn whether they have cancer, a new study found.

Women who have extremely dense breast tissue have a higher risk of breast cancer. The density of the tissue makes it difficult to detect the cancer with traditional mammography. That’s because dense tissue appears white on a mammogram and it’s hard for the radiologist to see through it when they read your scan. Nondense or fatty tissue appears black, and it is easier to distinguish between healthy tissue and cancer.

About 1 in 10 women are considered to have extremely dense breast tissue. More than half of women over the age of 40 in the United States have what’s considered dense breasts, according to the US Food and Drug Administration.

It’s unclear why women have this kind of tissue. Often, it’s inherited, but other factors can influence tissue density. Typically, women with extremely dense breasts tend to be fitter or have a lower BMI; they are often younger and may be taking hormone therapy to ease their menopause symptoms.

The study, published in The New England Journal of Medicine on Wednesday, found that using supplemental MRI screening in this patient population with extremely dense breast tissue, and who had normal results with mammography, saw significantly fewer interval cancers than the women in the study who were assessed using mammography alone during a two-year screening period.

Interval cancers are the cancers detected within 12 months after a mammographic screening in which findings are considered normal, essentially in the interval between scheduled screenings.

There was a 50% lower rate of interval cancers among the women in the MRI group. Among those who underwent a biopsy, 26% had breast cancer.

Specifically, the interval-cancer rate was 2.5 per 1,000 screenings in the MRI group, compared to 5.0 per 1,000 screenings in the mammography-only group in the study.

The research was done among more than 40,000 women in the Netherlands who were between the ages of 50 and 75. They were randomly assigned to either a group that received standard screening or a group that received additional MRI screening.

Breast cancer is the second most-common cancer among American women, regardless of race or ethnicity, behind only skin cancer, according to the US Centers for Disease Control and Prevention. More than 260,000 women in the US were diagnosed with breast cancer and more than 40,920 women died from breast cancer in 2018.

The American Cancer Society recommends mammograms every year between the ages of 45 and 54, and every other year after that for as long as a woman is in good health and expected to live another 10 years. Women younger than 45 should be given the choice to start annual mammograms as early as age 40, the organization says.

In the United States, federal law mandates health care facilities tell a patient in what density category her breasts fit, so they can know their related risks – but there is no additional recommendation for supplemental screening under current guidelines.

While doctors know additional screening could help, for a guideline to change, scientists would need to determine whether the additional screening actually helps a woman’s health and ultimately improves her overall survival rate. Looking at interval cancers, then, is one way of seeing whether a change in the standards could help, because essentially, the earlier a cancer is caught, the better a woman’s chance of surviving that cancer with a good health outcome.

In this study, it appears the majority of the cancers found in the MRI group were found at an early – presumably more treatable – stage. The study is, however, unable to determine whether the tumors would have otherwise been detected through regular screening, or if they would have ultimately threatened the patients’ survival.

An editorial that accompanied the study suggests that “many doctors” have been sending women with dense breasts for MRIs, even without this kind of evidence that it may help, but “until now it was unclear whether that additional testing had any value.”

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While the study shows MRI screening can lower the rate of interval cancers for this patient population, the editorial still points to what it labels a “dilemma.”

It suggests that while this additional screening could be beneficial, there is still the risk that additional tests could give a woman a false positive, which could put them at risk for having procedures that may not ultimately increase their chances of survival.