The growing use of routine mammograms over the past 30 years has done little to lower the death rate from breast cancer but has sharply increased the number of women who are wrongly diagnosed with the disease, a new study reported. (Rui Vieira/ASSOCIATED PRESS)

The routine use of mammograms has led to more than 1 million women being unnecessarily treated for breast cancer over the past three decades, according to the latest scientific report to cast skepticism on the effectiveness of the test.

The study, published Wednesday in the New England Journal of Medicine, concluded that nearly one-third of women diagnosed with breast cancer would never have developed the full-blown disease if left untreated.

Nevertheless, in such cases patients typically undergo invasive procedures such as surgery, radiation therapy, hormonal therapy and chemotherapy, said H. Gilbert Welch, a coauthor of the study and a professor at the Geisel School of Medicine at Dartmouth College. “These are major medical interventions and they’re certainly not something you would want to undergo if you didn’t need to,” he said.

The researchers also concluded that today’s sharply lower death rate for breast cancer is mainly due to factors such as improved treatments rather than early detection through mammograms.

The new study immediately fueled the already fierce debate over how often women should get mammograms, a controversy that has embroiled policymakers, politicians and physicians, as well as their female patients.

The American Cancer Society warned that the study “must be viewed with caution.” The American College of Radiology attacked the validity of the study, charging that it was spreading “misinformation” and “the cost may be lost lives.”

The report is the latest study to undermine the once-strong consensus that regular screenings are crucial to safeguarding women’s health. Just in the past two years, a major study of Norwegian breast cancer patients found that routine mammograms reduced the risk of dying from breast cancer by less than 10 percent. Another study found no effect on death rates when comparing European nations where screening became prevalent in the 1990s with those where it became widespread in the 2000s.

Even before those findings, in November 2009, a key federal panel revised its guidelines on mammograms to say that women should begin regular screenings at age 50 rather than age 40, and then get the exam every other year rather than annually.

However, that revision by the U.S. Preventive Services Task Force, made as Congress was crafting the new health-care law, was assailed not only by groups such as the American Cancer Society and the American College of Radiology but also by the Obama administration.

In 2010, lawmakers tweaked a mandate in the health-care law requiring insurers to cover preventive services recommended by the task force free of charge. The law specifies that when it comes to mammograms, insurers must follow the task force’s old guidelines.

Wednesday’s study analyzed changes in the rates of early- and late-stage breast cancer in the United States over time.

The authors’ premise was that if screening were effective, its growing use would result in greater detection of early-stage breast cancers and a corresponding drop in the prevalence of late-stage cancers.

Instead, they found that while the number of early-stage cancers doubled over the past three decades, the rate at which women were found to have late-stage cancer dropped by only 8 percent.

Welch speculated that as mammography technology has become more advanced, doctors are discovering breast lesions in such an early state of development it is virtually impossible to distinguish them from benign cell clusters.

Yet even as the screenings produce false positives, they fail to catch forms of breast cancer that develop rapidly, explaining why the more widespread use of screenings has done so little to curb the rate at which late-stage breast cancer is found.

“The sad fact is that there’s a subset of women who develop such an aggressive form of cancer it literally can’t be caught early,” said Welch.

Debra L. Monticciolo, a physician who chairs the American College of Radiology’s Quality and Safety Commission, questioned many of the study’s methods, including the data used to account for fluctuations in the underlying incidence of breast cancer due to factors like the use of hormones.

“It stuns me that this got through peer review,” said Monticciolo.

She added that as a physician, she found it hard to believe that such a large share of screenings produce false positives. If that were the case, there would be countless tales of miraculous recoveries by women who refuse traditional treatment after being diagnosed with early-stage breast cancer.

“I’ve never seen such a case. And there’s nothing like that in the literature,” said Monticciolo.

Most important, she said, new studies continue to confirm the benefits of screenings.

“Mammograms clearly save lives,” she said.

Archie Bleyer, a professor at the Knight Cancer Institute at Oregon Health & Science University who is the study’s other coauthor, said he wouldn’t dream of suggesting that women cease getting mammograms altogether.

“There are clearly women who are benefiting from doing this,” he said.

But he and Welch said their findings suggest that the message to women about mammograms needs to be more nuanced.

“We need to start telling the truth,” said Welch. “We’ve promoted this as if it’s the most important thing a woman can do for her health. . . . And the truth is that it’s a really close call.”

Women in their 40s who feel comfortable getting annual mammograms should not be dissuaded from doing so, he said. “But women who have never felt good about it, who felt coerced into the procedure, should feel equally good about not having it.”

Similarly, Welch said, women who do get screening mammograms and are found to have a tumor in a very early stage may want to consider alternatives to aggressive treatment — engaging in watchful waiting, for instance, or signing up for a clinical trial of drugs that may halt or slow the progression of cancer. Men have faced similar issues in deciding what to do about screening and treatment of prostate cancer, he said.

“We should tell women about the trade-offs and we should allow them to make their own decision,” he said.