MONDAY, March 18, 2013 (HealthDay News) -- Some women 50 and older may be able to have a mammogram every other year without increasing their risk of developing an advanced breast cancer, according to a large new study on nearly 1 million women.
The findings are published online March 18 in JAMA Internal Medicine.
The study conclusions drew mixed reactions, including criticism for its methods, from other experts. Debate about who should get screened with mammography, and how often, has been ongoing.
In the new research, women aged 50 to 74 who got a screening mammogram every other year had a risk of developing advanced-stage breast cancer that was similar to that of women in the same age range who got the test every year.
The researchers evaluated nearly 12,000 women with breast cancer and more than 922,000 without, said researcher Dr. Karla Kerlikowske, a professor of medicine, epidemiology and biostatistics at the University of California, San Francisco School of Medicine.
The investigators considered whether the women had high breast density and whether they were taking combination hormone therapy with estrogen and progestin, both considered risk factors for breast cancer. Dense breasts have more glandular tissue and less fatty tissue.
"If you undergo [mammography every two years], you are not any more likely to end up with some advanced breast cancer than if you have annual screenings," Kerlikowske said of this group of women.
Screening every two years -- or biennially -- was also associated with a lower risk of having a false-positive result, the researchers found. False-positives -- suspicions of cancer that turn out not to be after more testing -- can cause more expense and distress, experts note.
However, for women aged 40 to 49 who have extremely dense breasts, the researchers found biennial mammograms were linked with nearly a twofold increased risk of advanced-stage cancers and large tumors, and also a high risk of false-positive results.
In 2009, the U.S. Preventive Services Task Force, an independent panel of experts, issued its guidelines that women 50 to 74 at average risk should get a mammogram every two years. Women 40 to 49, the task force said, should talk with their doctor about the risks and benefits of screening.
The task force guidelines are at odds with those of many other organizations, including the American Cancer Society, which recommends annual screening beginning at age 40.
The task force guidelines consider only a woman's age, according to Kerlikowske. Her team decided to study the benefits and harms of screening based not just on age but also on breast density and hormone therapy use.
Women in the study were 40 to 74. Most diagnosed with breast cancer during the years studied, 1994 to 2008, were 50 or older. They typically had dense or very dense breasts.
The study has flaws, said Dr. Daniel Kopans, a professor of radiology at Harvard Medical School and senior radiologist in the breast imaging division of Massachusetts General Hospital. He is also a member of the American College of Radiology's Breast Imaging Commission.
The groups weren't identical, he said, and that would have been the best way to study the issue. Information is lacking, too, he said, on why some women got screened annually and others did not. "Those screened every year may have had different risk factors," he said.
"I would tell women it makes sense to get screened every year," Kopans said.
In a statement, the American College of Radiology (ACR) pointed to an analysis published in 2011 in the American Journal of Roentgenology finding that under the biennial model, about 6,500 more women annually in the United States would die of breast cancer.
Looking at early versus late-stage cancer is not the best way to judge the best interval for mammograms, according to the ACR statement. Rather, it said, researchers should look at such factors as tumor size and other markers of detecting cancers early.
Dr. Robert Smith, senior director of cancer screening for the American Cancer Society, said that the study is "not an accurate look at one year versus two years." For the study, annual was defined as intervals of nine to 18 months, for instance, and two years as more than 18 to 30 months.
Kerlikowske, in response, said this interval variability reflects real life.
Despite that criticism, Smith said the type of individualized screening studied in the new research is the direction that cancer prevention is headed. "If we can identify more clearly not only who will and who will not get cancer, but what those screening intervals are and if they can be screened safely at a longer interval, that would be good," he said.
"Some day we may be able to say certain women can have longer intervals," Smith said.
To learn more about breast cancer risk factors, visit the American Cancer Society.