Nov. 25 (Bloomberg) -- Mammograms may lead to unneeded treatments for breast cancer that might have regressed naturally, according to new research that revives a debate over frequent screening.

Doctors and public health officials have debated the risks and benefits of regular mammograms for years. While the common assumption is that finding small, easily treated tumors will prevent the development of larger, deadly cancers in the future, studies have yielded mixed results.

The report, published today in the Archives of Internal Medicine, found that the rate of cancer among women who received biannual mammograms over six years in four Norwegian countries was 22 percent higher than those who didn’t. That may mean that tumors in those who weren’t tested regressed without being treated, researchers said.

“Our findings simply provide new insight on what is arguably the major harm associated with mammographic screening, namely, the detection and treatment of cancers that would otherwise regress,” said the researchers led by Per-Henrik Zahl at the Norwegian Institute of Public Health‘s epidemiology department in Oslo.

Breast cancer is the most frequently diagnosed tumor in U.S. women, excluding skin cancer, and is second only to lung cancer in the annual number of deaths. Robert A. Smith, director of cancer screening of the American Cancer Society, said the conclusion that more than one in five invasive breast cancers may regress without incident if not detected by mammography “is nothing more than an overreaching leap in logic.”

The study was funded in part by the U.S. Department of Veterans’ Affairs.

Natural Regression

Spontaneous regression has occurred with other tumors, including melanoma, kidney, cervical and colon cancers, the researchers said. While there have been only 32 such reports in breast cancer cases, that doesn’t mean it is rare, they said. The vast majority of breast cancers are treated immediately and aggressively, with few tumors following a natural course.

The cancer society said the authors’ conclusions were flawed. The excess number of cancers found is simply a reflection of the lead time gained with mammograms, alerting women to tumors years before they would normally be detected, Smith said.

There is little evidence that breast cancer can regress, Smith said. Given that many cancers are missed during mammograms, there is plenty of opportunity to compare current to past films, he said in a statement. While there are harms from screening, including false positives and unnecessary biopsies, “the benefits of regular screening far outweigh these limitations.”

Not Life-Threatening

There is growing evidence that a “considerable proportion” of breast cancers aren’t life-threatening, like many cases of prostate cancer, wrote Robert Kaplan, from the University of California at Los Angeles School of Public Health and Franz Porzsolt, from the University of Ulm in Germany, in an editorial that accompanied the study.

If the researchers’ hypothesis about natural regression is correct, 20 percent of women who got biannual mammograms were treated unnecessarily, Kaplan and Porzsolt said. The other possibility is that women who didn’t get the tests had undetected cancer and missed a shot at early treatment, they said.

The spontaneous regression hypothesis is hard to rule out, Kaplan and Porzsolt wrote.

“If the spontaneous remission hypothesis is credible, it should cause a major re-evaluation in the approach to breast cancer research and treatment,” they said. “We must also consider the ethical concerns associated with over-diagnosis and over-treatment.”

To contact the reporter on this story: Michelle Fay Cortez in London at mcortez@bloomberg.net

Last Updated: November 25, 2008 00:01 EST