Mammography works: It can detect cancer. On that point, at least, most experts agree.

For the past two decades, however, doctors and researchers have been debating the details of the popular screening, including the best age to start and whether the risks of false alarms outweigh the benefits of catching cancer in its early stages.

Mammograms, or X-rays of the breast, are the best way to spot cancer early, when it’s easier to treat and before it’s big enough to be felt or cause symptoms. For women between the ages of 50 and 74, there’s general agreement that mammograms are a valuable screening tool. The American Cancer Society recommends yearly mammograms, and the U.S. Preventive Services Task Force recommends mammograms every two years.

But for women in their 40s, it’s not so clear cut. Mammograms may identify cancers that could turn lethal. But they can also flag disease that would never have been a problem, triggering a cascade of potentially unnecessary treatment and anxiety.

False positives are no less of a problem for women older than 50. But there is a comparatively smaller risk of breast cancer in the 40s versus for those 50 and older, said Dr. Robert Smith, a cancer epidemiologist and senior director of cancer control for the American Cancer Society.

Most organizations, including the American Cancer Society and the National Cancer Institute, recommend screenings begin at age 40. But in 2009 the U.S. Preventive Services Task Force, a government-backed panel of preventive-medicine experts, shook up the cancer world when it recommended the procedure every two years, starting at age 50. Women between 40 and 49 can benefit from screening, but they should make an individual decision in conjunction with their doctor, the task force said.

Some research showed the new advice didn’t change breast screening rates: In 2010, women in their 40s continued to show up for their mammograms. About 70 percent of women older than 40 reported having a recent mammogram. But should they?

Below, two experts explain the rationale behind the differing screening guidelines. Their responses have been edited.

———

Dr. Robert Smith, a cancer epidemiologist and senior director of cancer control for the American Cancer Society:

Regular mammograms starting at age 40 are a smart move.

Regular mammograms can be thought of as a form of insurance, said Smith, an adjunct professor of epidemiology at Emory University.

"It takes time and is a bit of a nuisance, but it protects you against high enough odds — but still unlikely — that something catastrophic will happen," he said.

The average woman’s risk of breast cancer was 1 in 12 in 1980; today it is 1 in 8. Breast cancer risk rises with age.

"Most of that risk is going to occur after 65; the odds of developing it as a 40-something is less than 2 percent, about 1 in 60 women during the decade, Smith said.

But if it does happen, it’s more likely to be treated if detected by a mammogram before symptoms develop, Smith said.

Smith agrees the high rate of false positives needs to be reduced, though they can’t be completely eliminated. Research shows "that anxiety (related to a diagnosis) is short-lived and doesn’t affect whether you come back for a mammogram or not," he said. "Women tell us they understand that false positives are a fact of life and have a much higher priority on finding it early than avoiding downsides of having to go through the experience."

———

Michael LeFevre, co-vice chair of the U.S. Preventive Services Task Force and a professor and vice chair in family and community medicine at the University of Missouri:

Mammograms are important for some in their 40s; it’s up to each woman to decide.

The emotional, physical and financial toll related to false positives needs to be considered, LeFevre said. For women who have a suspicious mammogram, "there’s lingering anxiety that hangs out there," LeFevre said. "We don’t think it’s enough to discourage a mammogram, but women who don’t want the pain or anxiety might wait until age 50 to start screening," he said. "Another woman, who can live with false positives and unnecessary biopsies, might want to maximize her chances. Then she can start screening at age 40."

In statistical terms, if 1,000 women, starting at age 40, are followed until death, 30 will die of breast cancer if no screening is done, LeFevre said. If they are screened every other year between age 50 and 75, the number of deaths falls from 30 to 23, dropping the death rate from 3 percent to 2.3 percent. If screening begins at 40 instead of 50, the number of deaths can be lowered by one — to 22 from 23 per 1,000. But it also results in 5,000 additional mammograms and 500 false positives. That means 1 of 2 women screened in their 40s get called back due to something on their mammogram that requires further attention. Thirty-three of those get biopsies, and one death will be averted, he said.

"The balance is favored toward mammography but only a small amount," LeFevre said. "We judged that to be a small benefit relative to the harms.

"One of most common risk factors for women in their 40s is family history. They might decide starting earlier rather than later because there is greater benefit. Women with no risk factors might say ‘it’s not worth it to me.’ But they should be able to make that decision rather than be told what to do."