flickr photo by -Reji http://flickr.com/photos/rejik/6920252489 shared under a Creative Commons (BY-NC-ND) license
Looking at the main studies the figures vary from:
180 women needing to be screened to catch 1 woman who might have a tumour that she might die from. However out of the of the same 180 women 3 women would be treated unnecessarily for a tumour that they would not die from. This means these women will have part or whole breast removed and will often receive radiotherapy and chemotherapy. (2012 Independent U.K. Panelheaded by Michael Marmot)
In the Cochrane review (Systematic reviews of primary research in human health care and health policy, and are internationally recognised as one of the highest standard in evidence based health care) figures are even higher:
2000 women needing to be screened to catch 1 woman who might have a tumour she might die from. However out of the same 2000 women 10 women would be treated unnecessarily for a tumour that they would not die from. This means these women will have part or whole breast removed and will often receive radiotherapy and chemotherapy.
The following is a simple language leaflet based on the Cochrane review of breast screening from the Nordic Cochrane Centre updated 2012.
The reason for varying figures in numbers of over diagnosed cancers and the numbers of women needed to be screened to catch even one cancer that could cause death is dependent on what age groups were included. For example younger women under 50 will benefit less (Tumours in this age group less likely to progress to cause death than older age groups), how frequent screening was undertaken e.g every year (more harm likely, because more indolent tumours found) versus every 3 years and statistical methods used.
A helpful and detailed guide on how to interpret data generally as well as on mammography screening can be found here
The above raises serious questions for patients, health practitioners and policy makers to consider:
a) Sideffects such as heart disease and cancer on those inappropriately treated plus financial burden on health system treating those sideffects in the narrow sense plus wider financial sense in terms of social care and lost work hours- all for patients who were never going to die from their tumour.
b) Psychological stress on patients and patient’s families and friends when tumours that could lead to death were never present. False positives are still to be considered in addition to this)
c) The cost of screening so many women to catch so few cancers that would actually cause death. It has been argued that the money would be better spent on prevention and other forms of help for those already with cancer
Issues with overdiagnosis are not limited to breast screening but pervade most areas of cancer and also many other areas of medicine. Just some examples are research suggesting 25% of lung cancers may be over diagnosed and 60% of prostrate cancer amongst others.