Mammographic breast cancer screening. Part II. Non-randomised comparisons: results similar to those of randomised trials.
Screening practices in the early 21st century may not have the same impact on breast cancer mortality than the small reduction seen in some randomised controlled trials. To determine whether non-randomised comparative studies can help to estimate the possible benefits of mass screening for breast cancer, we conducted a review of the literature using the standard Prescrire methodology. The most reliable non-randomised studies are those that combine historical and geographic comparisons, include follow-up data recorded for at least 10 years after the introduction of screening, and take into account the date of cancer diagnosis in mortality calculations. We identified eight such studies, all conducted in Scandinavian countries, in regions where screening was generally introduced during the 1980s and 1990s. Three studies conducted in Sweden showed no statistically significant reduction in all-cause mortality among women with breast cancer after the introduction of screening. One of the two Finnish studies (statistically the most powerful) and a Danish study showed a statistically significant decrease in breast cancer mortality after the introduction of screening. A study conducted in four Norwegian counties showed no significant reduction in breast cancer mortality after the introduction of screening. A meta-analysis of five of these studies, corresponding to a total of about 4.7 million woman-years, showed that the introduction of organised breast cancer screening among women aged 50 to 69 years was associated with about a one-sixth reduction in breast cancer mortality. Another study, published after this meta-analysis, was conducted throughout Norway and spanned more than 15 million woman-years. It showed that the introduction of organised screening was associated with about a one-quarter reduction in mortality attributed to breast cancer, equivalent to about 20 to 30 fewer breast cancer deaths per 10 000 women invited for screening. Overall, the results of these different studies are consistent with the small decrease in breast cancer mortality observed in randomised trials and their least stringent meta-analyses. This rather modest benefit must be weighed against the adverse effects of screening and treatment, which will be examined in a future issue.
D001943:Breast Neoplasms / Q000000981:diagnostic imaging* / Q000401:mortality / Q000628:therapy; D015340:Epidemiologic Research Design; D019317:Evidence-Based Medicine / Q000379:methods*; D005260:Female; D006801:Humans; D008327:Mammography / Q000009:adverse effects; D008403:Mass Screening / Q000379:methods*; D061214:Patient Safety; D011237:Predictive Value of Tests; D011379:Prognosis; D016032:Randomized Controlled Trials as Topic; D018570:Risk Assessment; D012307:Risk Factors