Screening for breast cancer appeared to have a very limited effect on the occurrence of serious and aggressive cancer cases. On the other hand, it appeared to detect many more early cancer cases, cases which would otherwise never have developed - but which are treated due to screening.
This is the conclusion of a study from Aarhus University, Denmark, that has just been published in the European Journal of Public Health based on data from all women over the age of 20 in Norway (approx. 1.8 million in 2010).
Looks at the various stages of cancer
The new element is that the researchers look at the severity of the diagnosis, which is divided into four stages: From the very early stages of cancer which is completely local up to the very serious cases where the cancer has already spread.
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," says Associate Professor, PhD Henrik Støvring, Aarhus University, who is the key researcher behind the project together with BSc Mette Lise Lousdal.
rimarily discovers indolent cancer
The researchers examined how the distribution of the four stages of cancer developed from 1987 to 2010: "We can see that since screening was introduced in Norway, the rate of discovery of breast cancer in the early stage among women aged 50-69 has almost doubled - while there has been virtually no change in the number of advanced stages. This suggests that screening primarily detects more cases of indolent cancer, which if there had been no screening, the woman would have died with - and not died of," says Henrik Støvring.
He adds that the screening may still have had a beneficial effect on mortality - this aspect was not examined by the study.
"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," he says.
The next step in the research project will be to analyse similar figures from Denmark.