Screening for breast cancer appeared to have a very limited effect on the number of serious and aggressive cancer cases. On the other hand, it appeared to detect cancer early in many more cases; cases that would otherwise never have developed were treated due to screening.

Those are the conclusions of a study from Aarhus University in Denmark recently published in the European Journal of Public Health (2014; doi:10.1093/eurpub/cku015). The study is based on data from all women older than 20 years in Norway (approximately 1.8 million in 2010). The study examined data from before screening was introduced in Norway (1987-1995), during its introduction (1996-2004), and after (2005-2010).

The new element is that the researchers look at the severity of the diagnosis, which is divided into four stages. The stages range from very early cancer that is completely local up to very serious cases where the cancer has already spread.

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The researchers examined the stage distribution of breast cancer diagnosed before the introduction of screening, during the introduction, and after the scheme was fully implemented.

“The idea of screening is that the cancer should be detected as early as possible so that the woman can be treated and cured. So when you introduce screening, women should be, as it were, transferred from having cancer in advanced stages to having cancer in an early stage. That is, if the screening works according to plan,” said senior author, Henrik Støvring, PhD, of Aarhus University.

The researchers examined how the distribution of the four stages of cancer developed from 1987 to 2010. Støvring said, “We can see that since screening was introduced in Norway, the rate of discovery of breast cancer in the early stage among women age 50 to 69 years has almost doubled, while there has been virtually no change in the number of advanced-stage cases. This suggests that screening primarily detects more cases of indolent cancer, which if there had been no screening, the woman would have died with but not died of.”

He added that the screening may still have had a beneficial effect on mortality—this aspect was not examined by the study. In considering if screening was beneficial relative to mortality, Støvring added, “But if that was the case then there should indeed be an increase of the early stages, but there ought to be an almost equally sized decline in the late stages as well. And this we did not find.”