A new study released today by Norwegian researchers offers an intriguing hypothesis: More frequent mammography screening intervals find some cancers that might otherwise "spontaneously regress" and would pose no threat were they not discovered.
The paper raises intriguing questions about the relationship between mammography screening and cancer rates, although at least one mammography expert says the authors may be misinterpreting a phenomenon that he believes often occurs with the start of a new mammography screening program. The study findings are published in today's issue of Archives of Internal Medicine (November 2008, Vol. 168: 21, pp. 2311-2316).
In the study, Dr. Per-Henrik Zahl, Ph.D., and colleagues at the Norwegian Institute of Public Health in Oslo examined breast cancer rates among 119,472 women ages 50 to 64 who received a screening mammogram every two years between 1996 and 2001 as part of a national breast cancer screening program that started in 1996. The women were drawn from four counties in Norway.
The team compared cancer rates among these women to those of a control group of 109,784 women who received just one screening mammogram after six years. The control group consisted of women who were between the ages of 50 and 64 in 1992, before Norway's screening program was implemented.
Zahl's team found that breast cancer rates were higher among the women in the study group than among the control group. Of every 100,000 women in the group with biennial screening, 1,909 had breast cancer during the period, while 1,564 of every 100,000 women had cancer in the control group, which was screened just once in six years.
The data suggest that some of the cancers detected by mammography may have "spontaneously regressed" had they not been discovered and treated, the team wrote.
"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," they wrote. "This raises the possibility that the natural course of some screen-detected invasive breast cancers is to spontaneously regress."
Zahl expanded on the concept in an e-mail to AuntMinnie.com, stating that screening mammography's ability to find very small tumors might be the reason for the phenomenon.
"Mammography detects many small tumors that are bound to spontaneously regress," Zahl wrote. "Regression is very common: two of three cancers detected at mammography would disappear in the absence of screening. It is of great biological interest."
Evidence of overtreatment?
Is the paper evidence that women are being overtreated as a result of screening mammography? Perhaps. But even if that were the case, there's no way to distinguish between "safe" and "dangerous" cancers detected on screening mammography, according to Dr. Daniel Kopans, professor of radiology at Harvard Medical School and senior radiologist in the Breast Imaging Division at Massachusetts General Hospital (MGH) in Boston.
"It is likely that there are some cancers that are not biologically dangerous, but, as the article admits, these are very uncommon, and we do not as yet have a way of identifying them," he said. "This does not mean that women should forego a potentially lifesaving study. We do not stop treating potentially life-threatening pneumonias with antibiotics because the majority of individuals will die without them, despite the fact that a small number of people may recover without antibiotics."
The study has some major problems, Kopans said, including comparing women from two different chronological periods, which introduces a bias that cannot be corrected with certainty. His belief is that when a mammography screening program is implemented, it initially finds more cancers: not only because it is finding the cancers that would have been found without screening, but also because it's detecting those cancers that would not have been found for one, two, three, or more years without screening.
So at the onset of a screening program, the incidence of breast cancer jumps until all women who are going to be screened are participating. The incidence then returns almost to the baseline before screening, with the same incidence as before screening, but with cancers being detected at a smaller size, Kopans said.
"It is entirely possible that the observations that have been made [in the study] are a function of the steady and unexplained increase in incidence and not of cancer regression," Kopans said. "It would be a major mistake to dissuade women from screening, since it has been shown that screening is saving thousands of lives."
Zahl and colleagues emphasize that their study results "have no bearing on the debate on whether screening mammography reduces breast cancer mortality."
"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," they wrote.
By Kate Madden Yee
AuntMinnie.com staff writer
November 24, 2008
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