3 jan. 2017
How women perceive the data from screening, influenced by optimistic slogans and presentations, versus reality:
When the authors of the report of the Swiss Medical Board (a Swiss medical commission independent from government authorities) evaluated the relevance of the mammographic screening program for breast cancer (see article), (PDF Article) they looked at one data that had been studied in several countries (4), namely how women perceive the benefits of screening according to what has been communicated about it and the information they have received, and that have made their beliefs on the subject.
The authors have presented a comparative table, with data gathered from the perception survey of American women in Part A, and real, objective data from the most probable scenarios, observed from the most convincing and among the most reliable studies in Part B (1-3)
The authors were astonished by the significant discrepancy between women's beliefs about the benefits of screening and reality.
The projected number of women in their fifties who would survive, develop breast cancer and die from other causes while doing regularly screening over 10 years, was compared to the expected number of women who would survive, develop breast cancer or die from other causes, and not doing screening.
71.5% of these interviewed American women estimated that screening mammography reduced by half the risk of dying from breast cancer, and 72.1% believed that at least 80 deaths would be prevented for every 1,000 women invited for screening.
The presentation of mortality risk reduction as a percentage embellishes the data.
A 20% reduction in mortality (a figure found on public institution websites, women committees websites and in the information brochures distributed to women) does not mean that 20 out of every 100 women will die of breast cancer, but that only one less woman will die of it in the best case (and without considering the other, more numerous women screened, who will at the same time suffer from overdiagnosis and false alarms).
The relative risk reduction of 20% corresponds to a comparison between the screened and unscreened groups. If, for example, 5 out of 1000 unscreened women die and 4 out of 1000 screened women die of breast cancer, the relative risk reduction resulting from the comparison of these two groups corresponds to this 20% ((5-4)/5=0.2 ), but in absolute terms it is only one woman who is saved. The data that the authors have collected for the Swiss population show also these optimistic expectations in a similar way.
The authors legitimately ask the question, how can women make an informed decision if the benefits of the screening program are overestimated?
We asked ourselves the same question…and tried to answer it (see the brochure on the home page).
1-Gotzsche PC, Jorgensen KJ. Screening for breast cancer with mammography. Cochrane Database Syst Rev 2013;6:CD001877-CD001877
2-Independent UK Panel on Breast Cancer ScreeningThe benefits and harms of breast cancer screening: an independent review. Lancet 2012;380:1778-1786
3-Miller AB, Wall C, Baines CJ, Sun P, To T, Narod SA. Twenty five year follow-up for breast cancer incidence and mortality of the Canadian National Breast Screening Study: randomised screening trial. BMJ 2014;348:g366-g366
4-Domenighetti G, D'Avanzo B, Egger M, et al. Women's perception of the benefits of mammography screening: population-based survey in four countries. Int J Epidemiol2003;32:816-821