Thirteen years of breast screening had no measurable effect on breast cancer mortality in Norway

Olsen et al. used largely the same data material and the same methods as those used by Kalager et al. to estimate the impact of breast screening in Norway on breast cancer mortality. Unsurprisingly, the results were very similar. Olsen et al. found a nonsignificant 11% reduction (95% confidence interval: a 23% reduction to a 12% increase) and Kalager et al. found a 10% reduction (95% confidence interval: a 24% reduction to a 4% increase). Both author teams compared screened and unscreened women using a contemporary and two historical control groups. Despite the nonsignificant difference, both teams nevertheless concluded that there was an association between mammography screening and reductions in breast cancer mortality. However, the further interpretation of these same findings differed radically. Kalager et al. argued that ‘‘. screening itself accounted for only about a third of the total reduction’’ because specialized treatment was introduced simultaneously with organized screening, and similar reductions were found in nonscreened age groups in the screened areas. Olsen et al. argued that prior regular mammography use in 40% of Norwegian women diluted the ‘‘true’’ effect of screening of 25% seen in the screening trials and that their result ‘‘corresponded very well’’ with expectations. A ‘‘true’’ effect of 25% is exactly the same as Olsen’s previous estimate of a 25% effect of breast screening in Copenhagen based on an observational study. We have published data with longer follow-up and shown that there is no measurable effect of breast screening in Copenhagen and Funen. Their previous publication from 2011, which measured mammography use in Norway, also exactly predicted the results in this article. They wrote: ‘‘Assuming that the Norwegian program would in absence of prior screening have decreased breast cancer mortality by 25%, and that the activity inand outside the organized program were equally effective, the measured effect of the organized program would under actual circumstances be a reduction of 11%. ’’ Such precision and predictive powers are astonishing, but their basic arguments do not hold. The assessment of prior mammography use is flawed and it is speculative whether opportunistic screening will ‘‘dilute’’ a screening effect. First of all, Olsen et al. do not differentiate between screening and clinical mammograms, neither in this article nor in their previous study to which they refer. In this article, they write that ‘‘40% of women used regular mammography prior to the program,’’ which suggests that 40% of Norwegian women had prior opportunistic screening mammograms. However, what they previously reported was that ‘‘Prior to start of the organized program, 40% of women in target age groups reported to have had mammography examination. ’’ This could be both screening and clinical mammograms, and there was no quantification of either type of mammogram in their report. This difference is crucial, because the idea with organized breast screening is to reduce breast cancer mortality above that which can be obtained by using mammograms on a clinical suspicion of breast cancer. Thirty-six percent of women in the screened age range in nonscreened areas of Denmark reported to have had a clinical mammogram previously, whereas only 5% had a screening mammogram, and 3% had both. There is no reason to think opportunistic screening is more common in Norway than in Denmark and some of the authors of the present article have previously shown that opportunistic screening in Denmark is rare. In that study, the authors did make the crucial distinction. Back then, in 2005, they argued that the only way to obtain widespread screening use would be through implementation of an organized programme. A national breast screening program was implemented in Denmark from 2007. Second, opportunistic breast screening largely takes place in private clinics or at diagnostic radiological departments, not in specialized screening units such as those used in the randomized trials or in organized screening programmes. The assumption by Olsen et al. that the effect of opportunistic screening on breast cancer mortality is ‘‘equal’’ to that of organized screening is speculative and unsupported by evidence and contrasts with the notion that screening should only be undertaken within the setting of an organized programme with rigid quality assurance as specified in guidelines. Olsen et al. note that screening has not led to a reduction in advanced breast cancers in Norway, but they do not draw the obvious conclusion that screening therefore cannot have reduced breast cancer mortality. The only conclusion that can be supported by the available data is that; ‘‘An association between breast screening in Norway and a reduction in breast cancer mortality could not be demonstrated. ’’ Thirteen years of breast screening in Norway have had no measurable effect on breast cancer mortality; it has not reduced the occurrence of advanced breast cancer either but has led to substantial overdiagnosis and increased Le tt er to th e E di to r