- The probability of having an unnecessary biopsy is higher with annual screening
- False-positive results are a source of anxiety and common in many mammograms
- Annual mammograms not conclusively more effective than biennial screenings
- Federal advisory group recommendeds most women get screened once every two years
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.