Mammography Invitation Equals 28% Reduction in Breast Cancer Death Risk

Mammography screening for breast cancer has a “substantial” mortality risk benefit

medwireNews: A prospective study of mammography screening in Norway points to a 28% reduction in the risk of breast cancer death for women who are invited to screening every 2 years compared with non-invitees.

However, the researchers note in the British Journal of Cancer that this “substantial effect” may not persist into the future, recognising that “evolving improvements in treatment will probably lead to a gradual reduction in the absolute benefit of screening.”

The study followed up Norwegian women between 1986 and 2009, during which time the national screening mammography programme was initiated. Over 15,193,034 person–years, 1,175 women invited to screening died of breast cancer, as did 8,996 women who were not.

This gave a mortality ratio for invitation to screening of 0.72 after adjusting for patient age, birth cohort, country of residence and underlying national trends in breast cancer death, report Harald Weedon-Fekjær, from the Norwegian University of Science and Technology in Trondheim, and co-authors.

However, the benefits weakened after screening ended, so that 5 to 10 years later, the mortality rate ratio was 0.79.

Further analysis calculated that 368 women aged 50 to 69 years must be invited to biennial screening to prevent one death from breast cancer. However, estimates for the approximate 76% of women who actually attended screening suggested the benefit rises to a 37% reduction in the risk of death, so that 280 women must be screened to prevent one death.

But the US authors of an accompanying editorial say that the Norwegian study confirms that the benefits of screening mammography are “modest at best” and note that the small benefits are accompanied by harms such as overdiagnosis, psychological stress and healthcare costs that are not always adequately presented to patients.

Joann Elmore, from University of Washington in Seattle, and Russell Harris, from University of North Carolina in Chapel Hill, say the Norwegian study’s findings “should make us reflect on how to monitor the changing benefits and harms of breast cancer screening.”

They add: “Future studies will hopefully allow analyses to account for changes over time in risk factors, screening technology, and treatment. Just as quality criteria have been defined for [randomised controlled trials], creative study methods and quality metrics must be developed for observational studies evaluating large screening programmes.”

References

Weedon-Fekjær H, Romundstad P, Vatten L. Modern mammography screening and breast cancer mortality: population study. BMJ 2014; 348: g3701 17 June. doi: http://dx.doi.org/10.1136/bmj.g3701

Elmore J, Harris R. The harms and benefits of modern screening mammography.Women need more balanced information. BMJ 2014; 348: g3824 17 June. doi: http://dx.doi.org/10.1136/bmj.g3824

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