In many countries, women are invited to regular X-ray screening of the breasts (mammography) to diagnose tumours before cancer cells spread to other organs.
A group of Danish and Norwegian researchers examined the effect of mammography screening in Denmark by looking at the size of breast tumours. Women in one group took part in a screening programme introduced in two Danish regions, while participants in the control group came from regions without established programmes.
Does not deliver expected benefits
The researchers found no fewer large (advanced) tumours in the group who participated in the screening programme than in the control group. Tumours of over two centimetres in diameter are likely to spread, if they have not already done so.
“This means that it is unlikely that the screening programme has given the benefits hoped for in terms of reduced mortality from breast cancer and a reduction in surgery,” says Per-Henrik Zahl from the Norwegian Institute of Public Health.
However, a 2015 evaluation of the Norwegian mammography screening programme by the Research Council of Norway showed that the target of 30 per cent lower mortality from breast cancer was almost reached and that there was an overdiagnosis of 10 to 20 per cent.
Zahl believes that if the Norwegian data had been analysed in the same manner as the Danish study, they would have reached the same result. There has been no decline in the incidence of metastatic tumours or large tumours in Norway either.
Found more small breast tumours
Zahl says that in Denmark they found a significant increase in the number of small breast tumours and several precursors that would never develop into clinical disease in the patient's lifetime or might even regress. This is called overdiagnosis.
The researchers also studied whether an increased incidence of breast cancer in the screening group was followed by fewer cases of breast cancer when women no longer participated in the screening programme. They found no such reduction.
“It means that screening does not detect tumours earlier. Finding several tumours in a screening programme does not indicate the tumours are being diagnosed earlier. All the increase is due to overdiagnosis,” says Zahl.
Jørgensen KJ, Gøtzsche PC, Kalager M, Zahl P. Breast Cancer Screening in Denmark: A Cohort Study of Tumor Size and Overdiagnosis. Ann Intern Med. [Epub ahead of print 10 January 2017]:. doi: 10.7326/M16-0270