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 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).
Examining the various stages of cancer
The new element is that the researchers looked at the severity of the diagnosis, which is divided into four stages, from the very early stages when cancer is local up to the metastatic phase.
In this study, 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 was, 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 of the Aarhus University, who is a lead researcher behind the project together with BSc Mette Lise Lousdal.
Primarily 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 added 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 authors concluded that incidence of localised breast cancer increased significantly among women aged 50-69 years old after introduction of screening, while the incidence of more advanced cancers was not reduced in the same period when compared to the younger unscreened age group.
The next step upon retrieving data from the Cancer Registry of Norway in the research project will be to analyse similar figures from Denmark.
Lousdal ML, Kristiansen IS, Møller B, et al. Trends in breast cancer stage distribution before, during and after introduction of a screening programme in Norway. Eur J Public Health; 2014 Mar 4. [Epub ahead of print]