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There is a number of misreading of the original texts1,2 in the comment from Kalager et al. We will therefore limit our response to the key points. Our Int J Cancer paper1 reports on breast cancer mortality in Norway following the start of the organized mammography screening program in 1996. We found that the breast cancer mortality had decreased by 7 to 11% after the start of the program. In the interpretation of this outcome, we use a previous review on mammography activity in Norway before and during the program.2 According to three independent data sources, a considerable proportion of Norwegian women had undergone mammography before start of the organized program. In this situation, the effect of the program on breast cancer mortality is expected to be smaller than it would have been in the absence of prior mammography.

Mammography can be used as a clinical examination or as a screening tool. In a public paid health care system with no organized screening program, the border between the two types of mammography depends on the payment system as well as on the clinical indication. Before the organized programs, Denmark had less mammography activity than Norway. In Denmark, as late as in 2000, only 3% of women aged 50–69 in non-program regions had a mammogram taken.3 On a 2-year basis, this will be considerably less than the 21% of Norwegian women in non-program regions in 1996 reporting a mammogram within the last 2 years.

Jørgensen et al. want to compare mammography activity in Norway and Denmark. This is a good idea. However, use of mammography also outside the organized programs has increased over time.3 A comparison therefore needs to be based on data from approximately the same calendar years. As shown in the answer to Kalager et al., above, prior to the organized programs, Denmark had less mammography activity than Norway. The possible difference in effect between organized and opportunistic screening was discussed in our mammography activity paper.2

Autier finds that the time trend in breast cancer incidence in Norway is incompatible with common use of mammography before the start of the organized program. As this point is discussed in-depth in our paper, we will not comment further on it.

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