WASHINGTON — More women are getting the word that they may have breasts too dense for mammograms to give a good picture. What is not so clear is what to make of that information.
This summer, New York became the fourth state to require that women be told if they have dense breasts when they get the results of a mammogram. That is because women whose breast tissue is very dense have a greater risk of developing breast cancer than women whose breasts contain more fatty tissue. Plus, it can be harder for mammograms to spot a possible tumor.
On Monday, scientists reported a bit of good news about yet another question: Do denser breasts also signal a worse chance of survival? A National Cancer Institute study tracked more than 9,000 breast cancer patients and concluded that those with very dense breasts were no more likely to die than similar patients whose breasts were not as dense.
‘‘It’s definitely reassuring,’’ said institute lead researcher Dr. Gretchen Gierach, who reported the results in the Journal of the National Cancer Institute.
Tumors might be found later in the most dense breasts, but once diagnosed they apparently were not more aggressive or harder to treat, explained coauthor Dr. Karla Kerlikowske of the University of California, San Francisco, who has long studied breast density.
‘‘That risk factor doesn’t affect her ability to respond to treatment, and treatment is good,’’ Kerlikowske said.
In fact, researchers were surprised to find an increased risk of death only in certain women with the least dense breasts — those who also were obese or had large tumors. Perhaps it has to do with increased hormones that accompany obesity, Gierach speculated, stressing that the finding needs further study.
Whatever the explanation, the research illustrates just how much more there is to learn about breast density’s complex role in cancer.
‘‘There’s a large proportion of women who have dense breasts, but most of those people don’t get breast cancer,’’ Kerlikowske cautioned.
Mammograms can show if a patient’s breasts are made up mostly of dense tissue — milk-producing and connective tissue — or of fatty tissue. Fatty tissue appears dark on the X-ray. Dense tissue appears white. So do potentially cancerous spots, meaning they can blend in.