(Health.com) -- Women who have a screening mammogram every other year are substantially less likely than those who opt for annual screening to experience false-positive results and biopsies that turn out to be unnecessary, according to a new study funded by the National Cancer Institute.
Using data from a nationwide network of breast-cancer registries, researchers analyzed more than 386,000 mammograms from about 170,000 women who began having the breast X-rays between 1994 and 2006.
Over 10 years of screening, the study estimates, 61% of women who have annual mammograms and 42% of women who have biennial mammograms will be called back at least once for a follow-up test that reveals they do not in fact have cancer.
The probability of having an unnecessary biopsy is similarly higher with annual screening. Depending on the age at which they begin screening, 7% to 9% of women who have annual mammograms and 5% to 6% of those who have biennial mammograms will have unnecessary biopsies over a 10-year period, according to the study, which appears this week in the Annals of Internal Medicine.
Annual mammograms were not conclusively more effective than biennial screening at identifying dangerous late-stage cancers, although a slightly higher percentage of women in the biennial group did develop these cancers.
The researchers caution, however, that the small number of women in the study who received a diagnosis of invasive breast cancer (4,492) prevents them from drawing firm conclusions about the effectiveness of one screening schedule over another. A larger study will be needed to clarify that point, they say.
The findings don't show that biennial screening is better than annual screening, or vice versa, but rather that false positives -- a source of anxiety for many women -- are common and "part of the process of screening mammography," says lead researcher Rebecca Hubbard, Ph.D., an assistant investigator at the Group Health Research Institute, the research arm of a nonprofit health plan in Seattle.
"I don't think there is any one right answer," Hubbard says. "I think it's a personal decision where every single woman has to think about what is her risk tolerance, how would she handle a false positive, and her own personal breast cancer risk."
Doctors and researchers have been debating the pros and cons of annual mammograms for years, but the question has taken on added urgency since 2009, when a federal advisory group recommended that most women have mammograms every other year beginning at age 50, rather than every year beginning at age 40.
The group, known as the United States Preventive Services Task Force, suggested that screening decisions should be based on a woman's individual risk as well as her "values" regarding the potential benefits and harms of more frequent screening.
Laura Esserman, M.D., the director of the breast care center at the University of California, San Francisco, says the new study validates the task force's recommendation and "confirms that biennial screening is the best way to go." (Esserman was involved in neither the task force nor the study.)
The increased risk of advanced cancer associated with biennial as opposed to annual screening is "so tiny that [it's] not even relevant," Esserman says. Mammography is primarily useful for catching slow-growing cancers, she says, while fast-growing cancers -- which are more common among younger women, and tend to cause a palpable mass in the breast -- grow so quickly that even annual mammograms could miss them.
But Daniel B. Kopans, M.D., a professor of radiology at Harvard Medical School, in Boston, says the implications of the study are not so clear-cut. Studies like this one that analyze large amounts of registry data have certain inherent limitations, even when mitigating factors such as age and family history of cancer are taken into account, he says.
Unlike studies in which women are randomly assigned to receive either annual or biennial screening, the method used by Hubbard and her colleagues can't rule out the possibility that the women who chose to get annual mammograms are different from their peers in unidentified ways that may have skewed the findings, Kopans says.
"The reason that oncologists have not been calling for a cessation to annual screening beginning at the age of 40 is because they know well that the best way for their therapies to cure breast cancer is to find it early," Kopans says. "It is, probably, not a good idea to set the clock back by lengthening the screening interval."
The study has some important shortcomings. For instance, relatively few of the women completed a full decade of annual or biennial mammograms, so the researchers had to resort to statistical modeling to estimate the 10-year probability of false positives and unnecessary biopsies. And most of the mammograms included in the study were traditional film-screen mammograms, not the digital mammograms now in widespread use.
It's not clear how the change in technology might affect the relevance of the findings. In another study in the same issue of the journal, Hubbard and her colleagues compared digital and film mammography. Both techniques were similarly effective, they found, although digital mammography was better at identifying tumors in extremely dense breast tissue and estrogen receptor negative tumors, both of which are more common among women in their 40s.
Hubbard stresses that the greater likelihood of false positives associated with more frequent mammograms shouldn't dissuade women from getting the tests altogether. "It's really important that women are getting screened, and that concern or anxiety about false positive results doesn't become a barrier for participating," she says.
Copyright Health Magazine 2011