Back in 2005, in a letter published in the British Medical Journal, two Norwegian researchers proposed a startling idea. Noting discrepancies in the number of tumors found depending on how often women are screened for breast cancer, they concluded that “for small invasive breast cancers, spontaneous tumor regression is not uncommon.”

In other words, some and perhaps many breast cancers detected by mammography might go away on their own.

Today, in a landmark study in the Archives of Internal Medicine, they offer strong evidence that their suspicions were right.

Per-Henrik Zahl, MD, PhD, and his colleagues at the Norwegian Institute of Public Health in Oslo compared two large groups of women. One, consisting of 119,472 women age 50 to 64, received mammograms every two years between 1996 and 2001. The second group, with about the same number of subjects, was screened just once in six years.

The women screened biannually, the researchers showed, tallied up 22 percent more breast tumors than those screened just once during the same period. For every 100,000 women who were screened regularly, 1,909 were diagnosed with invasive breast cancer over six years, compared with 1,564 women who were screened less often.

Those additional tumors, picked up by frequent mammograms, the authors believe, may well have appeared and then disappeared in the women screened less frequently.

Their conclusion, if it is confirmed, “should cause a major re-evaluation in the approach to breast cancer research and treatment,” Robert M. Kaplan, PhD, and Franz Porzsolt, MD, PhD, two experts in the field, write in an editorial accompanying the report.

Indeed, the new finding “highlights how surprisingly little we know about what happens to untreated patients with breast cancer,” Kaplan and Porzsolt acknowledge. “In addition to not knowing the natural history of breast cancer for younger women, we also know very little about the natural history for older women.”

Isolated cases do exist of breast tumors that have vanished on their own. Although such reports are rare, that doesn’t mean the occurrence is uncommon. First, a tumor that appears and then melts away is unlikely to be discovered. And when a tumor is found by mammography and then removed, doctors have no way of knowing whether it might have disappeared on its own if it had gone untreated.

Immunologists have long known that certain immune cells can target and destroy cancer cells. That may in fact be happening all the time, as renegade cells become cancerous and are ferreted out and destroyed.

As the editorial points out, much more needs to be learned about the natural history of breast cancer, and of other leading cancers, as well. But the findings could encourage some women diagnosed with breast cancer to adopt a strategy of watchful waiting rather than treatment.

For now, the new findings don’t alter current recommendations for mammograms. But they do point up a devilish uncertainty that women and their physicians will face when interpreting bad news. If a test detects a small invasive tumor, is it a real threat, or a cancer that might go away on its own?