A large study published Tuesday adds to the growing body of research concluding that screening mammograms save relatively few lives from breast cancer while discovering many cancers that wouldn't have caused problems if left alone.
"The more we screen for cancer, the more we find it. But we could have saved some of these women the angst of being told they have cancer," said Nancy Keating, a researcher at Harvard Medical School and senior author of the study, which examined decades of screening data.
Other recent studies also have found that mammograms often lead to unnecessary treatment, including a British Medical Journal study in February that followed 90,000 Canadian women over 25 years. But to date, that message hasn't resulted in fewer mammograms or changes in treatment—largely because scientists still can't tell which breast cancers can be safely left alone.
"The challenge is, we can't tell which are the aggressive cancers," said Dr. Keating.
Growing doubts about the benefits of mammograms prompted the U.S. Preventive Services Task Force to change its recommendations in 2009. Since then, it has urged women to get mammograms every other year starting at age 50, rather than annually at 40. The American Cancer Society and other cancer advocacy groups continue to recommend annual screenings starting at 40.
Many health-care policies encourage more screening. Several states now require doctors to tell women if they have dense breasts, which can make mammograms less accurate, and to discuss more high-tech options. A growing number of doctors are rated—and compensated—on the percentage of their patients who are up to date on screenings. And the Affordable Care Act requires insurers to make mammograms free to women without copays or deductibles.
The latest study, published in the Journal of the American Medical Association, took an especially long view of the data, examining randomized trials and observational studies on mammograms back to the 1960s to calculate the benefits and harms for women at various ages. Researchers concluded that annual mammograms reduced the overall death rate from breast cancer by about 19%. But they also discovered that about 19% of the breast cancers found when women undergo 10 years of annual mammograms are "over-diagnosed"—that is, they never would have caused problems if left alone.About 225,000 cases of breast cancer are diagnosed in the U.S. each year, and about 40,000 people die of it, according to the American Cancer Society.
Younger women had the least benefit, researchers found. They estimate that for every 10,000 women in their 40s who undergo annual mammograms for 10 years, 190 will be diagnosed with breast cancer. But only five of those women would avoid dying of breast cancer as a result of the screening. Of the remainder, about 25 would die despite being treated, and 36 would be treated unnecessarily because the cancer wouldn't have become life-threatening.
For women in their 50s, 10 breast-cancerdeaths would be averted for every 10,000 women screened annually for 10 years. For women in their 60s, 42 breast-cancer deaths would be averted. But as many as 137 women in their 50s, and 194 in their 60s would be diagnosed and treated unnecessarily.
The conclusion that some cancers are overtreated is controversial, and critics note that it is based on statistical estimates alone. Scientists can't ethically watch to see whether some breast cancers progress and some don't. Even precancerous changes, known as ductal carcinoma in situ, can become invasive cancers, so those are almost always treated aggressively.
Treatment typically involves a combination of surgery, radiation, chemotherapy and hormone therapy, with side effects that can last for years.
Still, many breast-cancer survivors say they are simply grateful that their cancer was treatable. And some experts stress that even if screening spurs some unnecessary treatments, it saves lives. "Over a decade or so, we prevent between 10,000 and 11,000 deaths," said Richard Wender, chief cancer control officer at the American Cancer Society. "The overwhelming odds for any one women to benefit are quite low, but overall, from a population perspective, it's one of our best tools in the war on cancer."
Scientists are working on ways to distinguish between breast cancers that are slow-growing and those that are fast-moving and lethal, and to better predict which women are at high risk for aggressive cancers so that screening and treatment can be more targeted.
In the meantime, many cancer experts urge women and their doctors to weigh their individual risks and preferences.
"There isn't a one-size-fits-all on mammograms," said Dr. Keating, who said she discusses all the pros and cons with her primary-care patients at Brigham and Women's Hospital.
"I have a lot of patients who say, 'I'm comfortable waiting,' " she said.
