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According to the results of a study published in the Archives of Internal Medicine, some early breast cancers may disappear on their own.

Screening mammography has made important contributions to the early detection of breast cancer and to improved breast cancer survival. Researchers have noticed, however, that the introduction of breast cancer screening programs has led to an increase in the number of breast cancer diagnoses, and that this increase has persisted over time. Because most breast cancers that are detected are treated, relatively little is known about how these screen-detected cancers would have progressed if they had not been detected by screening.

To explore the possibility that some screen-detected breast cancers will regress (go away) on their own, researchers evaluated the frequency of breast cancer in Norway before and after introduction of a breast cancer screening program.

The researchers compared two groups of women:

Screened group: These women were 50-64 years of age in 1996, the year when the mammography screening program began. During six years of follow-up, women in this group were invited for three rounds of mammographic screening.

Comparison group: These women were 50-64 years of age in 1992, before the screening program began. These women were also followed for six years, but were not invited to be screened until the end of the six-year follow-up period.

If all screen-detectable breast cancers persist or worsen over time, the frequency of breast cancer at the end of six years of follow-up would be similar in these two groups.

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What the researchers found, however, is that the number of breast cancer diagnoses was 22% higher among women in the screened group than among women in the comparison group. This suggests that some of the breast cancers in the comparison group spontaneously regressed during the follow-up period.

To account for the possibility that the single mammogram offered to the comparison group missed some breast cancers, the researchers also extended the follow-up period and assessed the number of breast cancers at eight years. By this time the comparison group had been offered a second mammogram. Even with this extended follow-up, however, there were more breast cancer diagnoses in the screened group than in the comparison group.

A limitation of this study is that it is not a randomized clinical trial. It’s possible that women in the comparison group differed from women in the screened group in ways that could influence the study results. The researchers compared several characteristics of the two study groups, however, and concluded that the two groups were generally similar with respect to breast cancer risk factors such as reproductive history.

To conclude, this study raises the possibility that some early breast cancers spontaneously regress. This suggests that screening mammography may lead to some degree of over-diagnosis of breast cancer (i.e. screening may detect some breast cancers that don’t need to be detected). Research into the possibility of spontaneous regression is still at an early stage, however. And even if spontaneous regression does occur, it is currently not possible to predict which cancers will regress. Breast cancer screening and treatment recommendations, therefore, remain unchanged.

Reference: Zahl P-H, Moehlen J, Welch G. The natural history of invasive breast cancers detected by screening mammography. Archives of Internal Medicine. 2008;168:2311-2316.

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