Editor—In this issue Zackrisson et al report on follow-up data from the Malmö mammographic screening trial and conclude that the rate of overdiagnosis of breast cancer was 10%.1 They do not, however, calculate the risk we believe is most relevant to women considering mammography: What is the chance that a screen detected cancer represents overdiagnosis?
After 15 years of follow-up, there were 1320 diagnosed in the screened group and 1205 in the control group (table 1). The excess detection of 115 cancers associated with screening led to their conclusion of an overdiagnosis rate of 10% (115/1205).
However, because the intervention had stopped 15 years earlier and yet breast cancer cases continue to accumulate in both groups, the approach understates the risk of overdiagnosis.
A more relevant denominator is the number of cancers found in the screened group at the end of the trial—741 (table 2). This addresses the question: Were I found to have cancer after being randomised to screening, how likely is it to represent overdiagnosis? As shown in the figure, using this denominator the risk of overdiagnosis is 15% (115/741).
Figure 1.
Risk of overdiagnosis in screened group at end of trial
However, many of the cancers detected in the screened group are not detected by screening. They are instead clinically detected (either during the interval between screening examinations or among non-attenders). The most relevant denominator is the number of screen detected cancers found at the end of the trial. This addresses the question: Were I found to have cancer by a mammogram, how likely is it to represent overdiagnosis?
Although this denominator is not reported by Zackrisson et al, the original BMJ article describing Malmö reported that 64% of the cancers detected in the screened group were detected by screening mammography.2 Thus one can deduce that the number of screen detected cancers at the end of the trial was about 475. As shown in the figure, using this denominator the risk of overdiagnosis is 24% (115/475).
See Research p 689
Competing interests: None declared.
References
- 1.Zackrisson S, Andersson I, Janzon L, Manjer J, Garne JP. Rate of over-diagnosis of breast cancer 15 years after end of Malmö mammographic screening trial: follow-up study. BMJ 2006;332: 689-92. (25 March.) doi: 10.1136/bmj.38764.572569.7C (published 3 March 2006). [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Andersson I, Aspegren K, Janzon L, Landberg T, Lindholm K, Linell F, et al. Mammographic screening and mortality from breast cancer: the Malmö mammographic screening trial. BMJ 1988;297: 943-8. [DOI] [PMC free article] [PubMed] [Google Scholar]