The article by Autier et al.1 shows a most elementary lack of understanding of screening and is simply wrong.
The authors’ assumption that, “during post-intervention periods, because screening (or absence of screening) activities are similar in the screening and in the control group, cancer detection rates in the two groups are also similar” is nonsense since a large number of cancers in the ASP will have been screen detected in the intervention period, which otherwise would have been detected in the post-intervention period. Thus, in the post-intervention period, cancer detection rates in the ASP will be lower than in the control group, and breast cancer mortality also will be lower. This is what they found, and is what they should expect if screening reduced breast cancer mortality.
This fundamental point also invalidates their argument against the closure screen of the control group; a closure screen of the control group is conservative, albeit less biased than the authors’ preferred method.2
When arguing against the closure screen, the authors’ adjusted estimates are wrong, since they subtract the deaths from cancers detected at screening of the control group, but not those detected contemporaneously in the study group. In the Two-County Trial, the reduction in mortality from breast cancers prior to the closure screen has been in the public domain since 1985,3 and was 31%, similar to the reduction in mortality observed when the control group includes deaths from cases diagnosed in the closure screen and their counterparts in the study group.4 Why resort to speculation when the empirical data are already published?
Their assumption that the 10% of breast cancer deaths in the control group in the Two-County Trial from cancers detected in the closure screen can be applied to other trials also is naïve, as is their argument that the smaller numbers of advanced cancers in the study group contemporaneously with the control group’s closure screen somehow invalidates the design and analysis. The PSP screen is a prevalent screen, whereas at that time, the ASP is in incident screen mode. The appropriate comparison is with the prevalent screen of the ASP, which was published in 1992 and shows similar results to the PSP closure screen.4
The criticism of the Swedish Two-County Trial on the grounds of imbalances in missing values is inaccurate, not only are we unable to verify these figures from the trial data, we cannot find them in the paper that Autier et al cite as the source.4
The issue of potential bias in cause of death has been examined time and again and shown to be a red herring.5–7 Indeed, the Swedish overview has published the excess mortality analysis which does not require classification of cause of death and found essentially the same mortality reduction as in the cause-specific analysis.8
The paper does not contribute to the debate on the value of mammographic screening, but confuses the discussion due to fatal errors that negate their conclusions.
Declarations
Competing interests
None declared
References
- 1.Autier P, Boniol M, Smans M, Sullivan R, Boyle P. Statistical analyses in Swedish randomised trials on mammography screening and in other randomised trials in cancer screening: a systematic review. J R Soc Med 2015. DOI: 10.1177/0141076815593403. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Duffy SW, Smith RA. A note on the design of cancer screening trials. J Med Screen 2015; 22: 65–68. [DOI] [PubMed] [Google Scholar]
- 3.Tabar L, Fagerberg CJG, Gad A, et al. Reduction in mortality from breast cancer after mass screening with mammography: Randomised trial from the Breast Cancer Screening Working Group of the Swedish National Board of Health and Welfare. Lancet 1985; i: 829–832. [DOI] [PubMed] [Google Scholar]
- 4.Tabar L, Fagerberg G, Duffy SW, Day NE, Gad A, Grontoft O. Update of the Swedish two- county program of mammographic screening for breast cancer. Radiol Clin Nth Amer 1992; 30: 187–210. [PubMed] [Google Scholar]
- 5.Tabar L, Fagerberg G, Duffy SW, Day NE. The Swedish two-county trial of mammographic screening for breast cancer: Recent results and calculation of benefit. J Epidemiol Comm Hlth 1989; 43: 107–114. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6.Tabar L, Duffy SW, Yen MF, Warwick J, Vitak B, Chen HHT, Smith RA. All-cause mortality among breast cancer patients in a screening trial: support for breast cancer mortality as an endpoint. J Med Screening 2002; 9: 159–162. [DOI] [PubMed] [Google Scholar]
- 7.Holmberg L, Duffy SW, Yen AMF, Tabar L, Vitak B, Nyström L, Frisell J. Differences in endpoints between the Swedish W-E (two-county) trial of mammographic screening and the Swedish overview: methodological consequences. J Med Screening 2009; 16: 73–80. [DOI] [PubMed] [Google Scholar]
- 8.Larsson LG, Nyström L, Wall S, et al. The Swedish randomised mammography screening trials: analysis of their effect on the breast cancer related excess mortality. J Med Screen 1996; 3: 129–132. [DOI] [PubMed] [Google Scholar]