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Frequent mammograms appear to detect breast cancer tumours that may otherwise spontaneously regress without treatment, suggests a new study, which indicates that breast cancer may be over-diagnosed and over-treated.

Norwegian researchers studied more than 200,000 women, roughly half of whom had mammograms every two years while the other half were screened only once after six years.

At the end of the study, they found that breast cancer rates were 22 per cent higher among the frequently screened group compared to those who were only screened once.

The researchers had previously believed that if every tumour detected were to progress, then incidence rates would fall with each round of mammograms. However, that was not the case with this study.

"Because the cumulative incidence among controls never reached that of the screened group, it appears that some breast cancers detected by repeated mammographic screening would not persist to be detectable by a single mammogram at the end of six years," the authors wrote. "This raises the possibility that the natural course of some screen-detected invasive breast cancers is to spontaneously regress."

The researchers published their findings in the November 24 edition of the Archives of Internal Medicine.

The researchers conceded that the medical community may have trouble embracing their findings.

However, previous research has found spontaneous tumour regression in some breast cancer patients. While the incidence rate in that study was small - 32 cases - the authors argue that because regression is rarely reported, it doesn't mean it rarely occurs.

The study raises the question of "whether some of those tumours, if left alone, will end up being overcome by the human immune system and not be life-threatening," Dr. Robert Kaplan of UCLA, who wrote an accompanying editorial commenting on the findings, told Â鶹´«Ã½.

"If this is correct, it will make something like watchful waiting a reasonable alternative for some women," Kaplan said.

If the study's findings hold up in future research, Kaplan said, it could mean that doctors are actually over-treating some cancer patients.

As mammogram-based screening programs have grown throughout North American and Europe, so too have breast cancer diagnosis rates.

But finding more cancer does not mean that doctors are finding more cancer that will potentially lead to serious illness and death.

If researchers can find a way to determine which tumours may regress and which ones may grow, then some women may be able to avoid the many unpleasant side effects of cancer treatment, from hair loss and fatigue to cognitive difficulties.

However, scientists do not yet know how to do this, Kaplan said.

"Most women will not want to take the gamble that they will be the one in five where the tumour goes away."

With a report by CTV's medical specialist Avis Favaro and senior producer Elizabeth St. Philip

The Natural History of Invasive Breast Cancers Detected by Screening Mammography

Per-Henrik Zahl, MD, PhD; Jan Moehlen, MD, PhD; H. Gilbert Welch, MD, MPH

Background: The introduction of screening mammography has been associated with sustained increases in breast cancer incidence. The natural history of these screen-detected cancers is not well understood.

Methods: We compared cumulative breast cancer incidence in age-matched cohorts of women residing in 4 Norwegian counties before and after the initiation of biennial mammography. The screened group included all women who were invited for all 3 rounds of screening during the period 1996 through 2001 (age range in 1996, 50-64 years). The control group included all women who would have been invited for screening had there been a screening program during the period 1992 through 1997 (age range in 1992, 50-64 years). All women in the control group were invited to undergo a 1-time prevalence screen at the end of their observation period. Screening attendance was similar in both groups (screened, 78.3%, and controls, 79.5%). Counts of incident invasive breast cancers were obtained from the Norwegian Cancer Registry (in situ cancers were excluded).

Results: As expected, before the age-matched controls were invited to be screened at the end of their observation period, the cumulative incidence of invasive breast cancer was significantly higher in the screened group than in the controls (4-year cumulative incidence:1268vs810per100 000 population; relative rate, 1.57;95%confidenceinterval,1.44-1.70). Even after prevalence screening in controls, however, the cumulative incidence of invasive breast cancer remained 22% higher in the screened group (6-year cumulative incidence: 1909vs1564per100 000population; relative rate, 1.22;95%confidence interval, 1.16-1.30). Higher incidence was observed in screened women at each year of age.

Conclusions: Because the cumulative incidence among controls never reached that of the screened group, it appears that some breast cancers detected by repeated mammographic screening would not persist to be detectable by a single mammogram at the end of 6 years. This raises the possibility that the natural course of some screen-detected invasive breast cancers is to spontaneously regress.