While it's often suggested that women with dense breasts get ultrasound or other extra screenings after a mammogram, a new study suggests those added tests may only help certain women.

Breast density increases breast cancer risk and can mask tumors during mammograms, the researchers explain in the Annals of Internal Medicine.

In some states, healthcare providers must tell women if they have dense breasts. Those women sometimes then go on to have additional screenings, such as ultrasound scans or magnetic resonance imaging (MRIs).

But in the new study, the researchers found that not all women with dense breasts have a high enough risk for breast cancer after a normal mammogram to justify having more screening tests.

Instead, they found that two specific groups of women with dense breasts were likely to benefit from extra screening, taking into account their risk for breast cancer in the next five years as calculated using an online tool called the Breast Cancer Surveillance Consortium (BCSC) score (bit.ly/1PuxYZF).

The first group consists of women with extremely dense breasts and a BCSC-calculated five-year breast cancer risk of 1.67 percent or greater.

The second group consists of women with heterogeneously dense breasts and a BCSC-calculated five-year risk of 2.50 percent or greater.

Women in these specific groups, which represent about a quarter of women with dense breasts, should discuss whether extra screening may be appropriate in addition to a mammogram every two years as recommended for women are 50 to 74, the authors write.

"Our paper helps women in that it really identifies people for whom screening is most important," said lead author Dr. Karla Kerlikowske of the San Francisco Veterans Affairs Medical Center. "Some don’t have to be concerned."

While those additional tests may help in finding tumors, the researchers warn they may increase the risk of so-called false-positives, which may lead to biopsies.

For the new study, researchers analyzed data from more than 300,000 women age 40 to 74 with no history of breast cancer or breast implants. The women received digital mammograms between 2002 and 2011, when their breast density was recorded.

The researchers also calculated each woman’s five-year breast cancer risk using the BCSC online tool, which takes into account age, race, family history of breast cancer, breast density, and whether or not a woman has had a breast biopsy in the past.

Available evidence suggests that women with dense breasts may be 1.2 to 2 times more likely to be diagnosed with cancer than those with average density, but since up to half of women have dense breasts, it is difficult to call density a “risk factor” for cancer, according to Dr. Nancy Dolan of Northwestern University Feinberg School of Medicine in Chicago, who coauthored an editorial accompanying the new study.

In the U.S. currently, most women who do not have a high lifetime risk of cancer would have to pay out of pocket for an MRI in addition to their recommended mammograms, which likely limits widespread use, Dolan told Reuters by email.

“Supplemental screening increases the rates of biopsy, cost and patient anxiety,” Dolan said. “Even among (women with) above average risk, supplemental screening with ultrasound has a very high false positive rate compared to mammography.”

The new study provides compelling evidence that breast density should not be the sole criteria to guide decisions about supplement screening, she said.