A little-noticed Scandinavian study is adding an intriguing new wrinkle to Canada's debate over breast-cancer screening, suggesting that many tumours found by mammograms - and often treated with surgery - would have simply vanished on their own.

A leading Canadian breast cancer scientist is touting the research as a possible paradigm shift in medicine's understanding of the disease, but acknowledges the study's provocative conclusions and their fascinating implications will divide the scientific community.

"[It] goes contrary to just about everything we think we know about breast cancer," said Dr. Steven Narod, the world's most-cited breast-cancer researcher. "It's a question that will at this point polarize the scientific community into the believers and the non-believers. The implications are extraordinary."

The notion that many cancers disappear without treatment calls into question the value of some screening and points to potential new ways of tackling the more lethal cases, said Dr. Narod, who practises at Toronto's Women's College Hospital.

The long-standing, emotional debate over when to start screening women for breast cancer with regular mammograms flared up again last month, when an independent task force recommended against routine tests for women under 50. The panel said screening creates too many risks - false positives and unnecessary treatment of non-life-threatening malignancies - while bringing relatively little benefits in terms of saving lives in those 40-something women.

Critics, however, said the task force used outdated evidence that underestimated the benefits and exaggerated the harms of screening.

Earning little attention at the time was a study published in October in the journal Lancet Oncology by PerHenrik Zahl of the Norwegian Institute of Public Health and colleagues.

The team looked at two large, age-matched groups of women in Sweden. Starting in the late 1980s, one cohort of about 300,000 received annual mammograms for six years. A control group underwent no screening for the first four years, then had mammograms in the last two.

Conventional thinking would suggest that the cumulative number of cancers found in both groups would be about the same at the end of the six years. Surprisingly, however, the outcome was quite different: The cohort of women who were screened annually - and likely treated when a tumour was found - had accumulated significantly more tumours - 174 more per 100,000.

The only explanation for why the unscreened women would have fewer cancers, the authors concluded, is that many breast tumours appear, then spontaneously just go away.

"It could be some kind of immune response," said Dr. Zahl in an interview Tuesday. "When a tumour goes in regression, there is something outside that is happening to it."

He pointed to a 2007 case study by Slovakian doctors that may represent a dramatic illustration of the possible phenomenon. A woman was diagnosed with a relatively large breast tumour but had her surgery delayed because of a car accident. A month later, the cancer had somehow disappeared.

Another top researcher in the breast cancer field has dismissed the Swedish-Norwegian findings, noting that they came from an observational study involving realworld screening programs, rather than a controlled trial in which researchers randomly assign subjects to one group or another, considered the most-reliable kind of medical evidence.

The groups of Swedish women were in separate regions of the country and any number of differences between them could have explained the divergent numbers of cancers, said Dr. Martin Yaffe, an expert on diagnostic imaging at Toronto's Sunnybrook Research Institute.

"If this was evidence presented in a court case, I think it would be thrown out," he said. "It's extremely speculative."

Regardless, there is no other, more direct evidence that any cancers simply "melt away" on their own, as intriguing as the idea seems, Dr. Yaffe said.

Dr. Zahl disagrees, saying there is research suggesting "regression" in cases of childhood neuroblastoma and melanoma, as well.

He concedes that it is unclear why some cancer might simply die off on its own, but the explanation likely lies with the impact of the patient's immune system on the tumour, he said.

If the Scandinavian conclusions prove true, examining the qualities of the vanishing cancers could point to new ways of treating the more lethal ones, Dr. Narod said, noting that he is now investigating research along those lines.

Both he and Dr. Zahl argued that the conclusions also call into question the value of regular mammogram screening of some women, if many of the tumours found end up evaporating. The Norwegian scientist goes further, suggesting that women diagnosed with the smallest tumours could in some cases be given minimal treatment with drugs like Tamoxifen and monitored to see if the tumour progresses - similar to the "watchful waiting" approach sometimes used with prostate cancer.

Dr. Narod, however, said the strategy would be too risky with breast cancer, given it is impossible now to distinguish between "the bad actors and the good actors" - the tumours that will keep growing and cause death if not treated, and those that, perhaps, will disappear if left alone.