Does screening mammography over-diagnose breast cancer?

Richard Heywood, University of Cambridge School of Clinical Medicine, Addenbrooke's Hospital, Hills Road, Cambridge, CB2 0SP

Breast cancer is the 2nd most common cancer worldwide (1), and the high prevalence has lead to the introduction of mammography screening programmes in many Western countries. The introduction of mammography has been hailed for saving lives, but there is still a concern that it may over-diagnose breast cancer, leading to unnecessary medical and surgical treatment, which can lead to physical and psychological side effects (2,3). In the latest issue of Lancet Oncology, Zahl et al (4) study the incidence of breast cancer in women before and after screening programmes in Sweden, and whether over-diagnosis is a problem.

The authors looked at over 370,000 women aged 40-74 before the introduction of screening and afterwards. They found a 35% increase in reported breast cancer incidence after the screening programmes were introduced compared to non-screened women. After the initial peak, it would be expected by the 3rd screening (6 years) that the control group incidence would catch up with the screened group, as the tumours diagnosed early on by screening would become clinically apparent later on in the control group. However, the cumulative incidence at 6 years was still higher in the screened group, suggesting some over-diagnosis of tumours, but also more provocatively suggesting that some of the tumours diagnosed by early screening would in fact regress, and never become clinically apparent without screening.

The authors estimate that 35% of “tumours” identified by screening are over-diagnosed, and that half of screening-detected tumours would undergo spontaneous regression, avoiding the need for treatment.

These findings question the utility of screening, and are particularly relevant given the recent announcement from the National Clinical Director for Cancer, Sir Mike Richards, that there will be an independent review to assess the risks and benefits of screening, and whether it should be continued in the UK (5).

References: 

1. Ferlay J, Shin H-R, Bray F, Forman D, Mathers C, Parkin DM. Estimates of worldwide burden of cancer in 2008: GLOBOCAN 2008. Int. J. Cancer 2010;127(12):2893-2917.
doi: 10.1002/ijc.25516

2. Maguire GP, Lee EG, Bevington DJ, Küchemann CS, Crabtree RJ, Cornell CE. Psychiatric problems in the first year after mastectomy. Br Med J 1978 Apr;1(6118):963-965.
3. Lostumbo L, Carbine NE, Wallace J. Prophylactic mastectomy for the prevention of breast cancer. Cochrane Database Syst Rev 2010;(11):CD002748.
doi: 10.1002/14651858.CD002748.pub3

4. Zahl P-H, Gøtzsche PC, Mæhlen J. Natural history of breast cancers detected in the Swedish mammography screening programme: a cohort study. Lancet Oncol. 2011 Nov;12(12):1118-1124.
doi: 10.1016/S1470-2045(11)70250-9

5. Richards M. An independent review is under way [Internet]. BMJ 2011 Oct;343Available from: http://www.bmj.com/content/343/bmj.d6843.short
doi: 10.1136/bmj.d6843

Story image from Wikimedia Commons.