Editor—Unfortunately, Zahl and Mæhlen have misunderstood the design of our study, which is essential for the correct interpretation of the data. Our analysis is not a simple comparison of breast cancer mortality in Copenhagen before and after screening was introduced, but it also uses the data from the rest of Denmark outside the screening regions to adjust for the underlying time trend in breast cancer mortality. Using a similar model for the breast cancer mortality in women aged 40-49 in Copenhagen in the screening period gives a relative risk of 0.94 (95% confidence interval 0.69 to 1.27). Breast cancer mortality in women aged 40-49 in Copenhagen did therefore not fall in the same way as that in women aged 50-69.
Warnings against use of hormone replacement, as mentioned by Grant, were issued mostly after our study period. A possible effect of this would furthermore be controlled for in our model, where we adjust for the underlying time trend in breast cancer mortality.
Gøtzsche et al say that we have not provided data on the possible harms of mammography screening. This is not correct as our paper includes a paragraph on the programme's short term performance indicators. Readers interested in incidence of breast cancer after screening was introduced are referred to our original paper.1 The effect of mammography screening in the Copenhagen programme reached significance after six years. In an overview of the Swedish randomised trials, the effect started to emerge about four years after randomisation.2
Competing interests: None declared.
References
- 1.Olsen AH, Jensen A, Njor SH, Villadsen E, Schwartz W, Vejborg I, et al. Breast cancer incidence after the start of mammography screening in Denmark. Br J Cancer 2003;88: 362-5. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Nyström L, Andersson I, Bjurstam N, Frisell J, Nordenskjöld B, Rutqvist LE. Long-term effects of mammography screening: updated overview of the Swedish randomised trials. Lancet 2002;359: 909-19. [DOI] [PubMed] [Google Scholar]