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Danish Study on Effect of Screening on Breast Cancer Mortality

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Key Points

  • Analyses including all breast cancer mortality and mortality diagnosed after first invitation to screening showed 10% and 11% reduction in risk with screening.
  • Analysis including only women diagnosed during screening eligibility showed a 20% reduction in risk.

In a Danish study reported in the Journal of Clinical Oncology, Beau et al used different models to assess the effects of screening on breast cancer mortality, finding a 20% reduction among patients diagnosed during the recommended screening age range. As noted by the investigators, the study was intended to explore the finding that a greater mortality benefit is observed in randomized trials of screening than in observational studies.

Study Details

The study included 18,781,292 person-years of data from Copenhagen and Danish registries, with 976,743 person-years from women invited to screening and 17,804,549 from control subjects. Women aged 50 to 69 years were invited to screening every 2 years, with adoption of the screening program starting in Copenhagen in 1991. The program was implemented gradually over several years in different regions, and the national policy started at the end of 2007.

Data were examined using 3 models: a “naive model” including all breast cancer deaths, a “follow-up model” including only breast cancer deaths in women diagnosed after their first invitation to screening, and an “evaluation model” that counted only breast cancer deaths and person-years in women diagnosed during screening age.

Modeling Outcomes

In the naive model, the breast cancer mortality rate was 130.0 per 100,000 person-years in the screening invited population compared with 148.2 per 100,000 person-years estimated in the absence of screening, with an age-adjusted rate ratio of 0.90 (10% risk reduction; 95% confidence interval [CI] = 0.84–0.97). In the follow-up model, the breast cancer mortality rate was 85.4 per 100,000 person-years compared with 100.7 per 100,000 person-years estimated in the absence of screening, with an age-adjusted rate ratio of 0.89 (11% risk reduction; 95% CI = 0.82–0.98).

In the evaluation model, excluding breast cancer deaths in women with disease diagnosed after they were no longer eligible for screening, the breast cancer mortality rate was 73.2 per 100,000 person-years compared with 93.0 per 100,000 person-years expected in the absence of screening. The age-adjusted rate ratio was 0.80 (20% risk reduction; 95% CI = 0.71–0.90).

The investigators concluded, “Screening had a clear long-term beneficial effect, with a 20% reduction in breast cancer–associated mortality in the invited population. However, this effect was, by nature, restricted to breast cancer deaths in women who could potentially benefit from screening. Our study highlights the complexity in evaluating the long-term effect of breast cancer screening from observational data.”

The study was supported by the Independent Research Fund of Denmark, Kirsten and Freddy Johansen Fund, and Fabrikant Vilhelm Pedersen og hustrus Fund.

Anna-Belle Beau, of the Department of Public Health, University of Copenhagen, is the corresponding author for the Journal of Clinical Oncology article.

The content in this post has not been reviewed by the American Society of Clinical Oncology, Inc. (ASCO®) and does not necessarily reflect the ideas and opinions of ASCO®.


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