Cancer researchers have known for years that it was possible in rare cases for some cancers to go away on their own. There were occasional instances of melanomas and kidney cancers that just vanished. And neuroblastoma, a very rare childhood tumor, can go away without treatment.

But these were mostly seen as oddities ?an unusual pediatric cancer that might not bear on common cancers of adults and a smattering of case reports of spontaneous cures. And since almost every cancer that is detected is treated, it seemed impossible to even ask what would happen if cancers were left alone.

Now, though, researchers say they have found a situation, in Norway, that has let them ask that question about breast cancer. And their new study, published Tuesday in the Archives of Internal Medicine, suggests that even invasive cancers may sometimes go away without treatment and in larger numbers than anyone ever believed.

At the moment, the finding has no practical applications since no one knows whether a cancer that is detected might disappear or continue to spread or kill.

And some remain unconvinced.

揟heir simplification of a complicated issue is both overreaching and alarming,?said Robert A. Smith, director of breast cancer screening at the American Cancer Society.

But others, including Robert M. Kaplan, chairman of the department of health services at the School of Public Health at the University of California, Los Angeles, are persuaded by the analysis. And, Dr. Kaplan said, the implications are potentially enormous.

If the results are replicated, Dr. Kaplan said, it might eventually be possible for some women to opt for so-called watchful waiting, monitoring a tumor in their breast to see if it grows. 揚eople have never thought that way about breast cancer,?he added.

Dr. Kaplan and his colleague, Dr. Franz Porzsolt, an oncologist at the University of Ulm, writing in an editorial that accompanied the study, said: 揑f the spontaneous remission hypothesis is credible, it should cause a major reevaluation in the approach to breast cancer research and treatment.?/P>

The study, by Dr. H. Gilbert Welch, a researcher at the VA Outcomes Group in White River Junction, Vt., and Dartmouth Medical School; Dr. Per-Henrik Zahl of the Norwegian Institute of Public Health; and Dr. Jan Maehlen of Ulleval University Hospital in Oslo compared two groups of women, all aged 50 to 64, in two consecutive six-year periods.

One group of 109,784 women was followed from 1992 until 1997. Mammography screening in Norway was initiated in 1996. In 1996 and 1997, all were offered mammograms and nearly every woman accepted.

The second group, of 119,472 women, was followed from 1996 until 2001. All were offered regular mammograms and nearly all accepted.

It might be expected that the two groups would have roughly the same number of breast cancers, either detected at the end or found along the way. Instead, the researchers report, the women who had regular routine screens had 22 percent more cancers. For every 100,000 women who were screened regularly, 1,909 were diagnosed with invasive breast cancer over six years as compared with 1,564 women who did not have regular routine screening.

There are other explanations, but researchers say that they are less likely than the conclusion that the tumors disappeared.

The most likely explanation, Dr. Welch said, is that 搕here are some women who had cancer at one point and who later don抰 have that cancer.?

The finding does not mean mammograms caused breast cancer. Nor does it bear on the question of whether women should continue to have mammograms ?since so little is known about the progress of most cancers.

Mammograms save lives, Dr. Smith said, adding that even though they can have a downside, most notably, he said, the risk that a woman might have a biopsy to check on an abnormality that turns out not to be cancer, 搕he balance of benefits and harms is still considerably in favor of screening for breast cancer.?/P>

But Dr. Suzanne W. Fletcher, a professor emerita of ambulatory care and prevention at Harvard Medical School, said it was also important for women and doctors to understand the entire picture of cancer screening and the new finding, she said, 搃s part of the picture.?

揟he issue is the unintended consequences that can come with our screening,?Dr. Fletcher said, meaning downsides, including biopsies for lumps that were not cancers or, it now appears, sometimes treating a cancer that might not have needed treatment. 揑n general we tend to underplay them,?she added.

Dr. Welch said the cancers in question had broken through the milk ducts, where most breast cancers begin, and invaded the breast. Such cancers are not microscopic, often are palpable, and are bigger and look more ominous than those confined to milk ducts, so-called ductal carcinoma in situ, or DCIS, Dr. Welch said. Doctors surgically remove invasive cancers and, depending on the circumstances, may also treat women with radiation and, sometimes, chemotherapy.

The study抯 design was not perfect, but researchers say the ideal study is infeasible. It would entail screening women, randomly assigning them to have their screen-detected cancers treated, or not, and following them to see how many untreated cancers went away on their own.

But, they said, they were astonished by the results.

揑 think everybody is surprised by this finding,?Dr. Kaplan said. He and Dr. Porzsolt spent a weekend reading and re-reading the paper.

揙ur initial reaction was, 慣his is pretty weird,挃 Dr. Kaplan said. 揃ut the more we looked at it the more we were persuaded.?BR>