Understanding systematic reviews: the meta-analysis graph (also called ‘forest plot’)

Abstract

Background

Mammographic screening for breast cancer is controversial, as reflected in greatly varying national policies.

Objectives

The objective was to assess the effect of screening for breast cancer with mammography on mortality and morbidity.

Search strategy

MEDLINE (16 May 2000), The Cochrane Breast Cancer Group’s trial register (24 Jan 2000) and reference lists. Letters, abstracts and unpublished trials. Authors were contacted.

Selection criteria

Randomised trials comparing mammographic screening with no mammographic screening.

Data collection and analysis

Data were extracted by both authors independently.

Main results

Seven completed and eligible trials involving half a million women were identified. The two best trials provided medium-quality data and, when combined, yield a relative risk for overall mortality of 1.00 (95% CI 0.96–1.05) after 13 years. However, the trials are underpowered for all-cause mortality, and confidence intervals include a possible worthwhile effect as well as a possible detrimental effect. If data from all eligible trials (excluding flawed studies) are considered then the relative risk for overall mortality after 13 years is 1.01 (95% CI 0.99–1.03). The best trials failed to show a significant reduction in breast cancer mortality with a relative risk of 0.97 (95% CI 0.82–1.14). If data from all eligible trials (excluding flawed studies) are considered then the relative risk for breast cancer mortality after 13 years is 0.80 (95% CI 0.71–0.89). However, breast cancer mortality is considered to be an unreliable outcome and biased in favour of screening. Flaws are due to differential exclusion of women with breast cancer from analysis and differential misclassification of cause of death.

Reviewer’s conclusions

The currently available reliable evidence does not show a survival benefit of mass screening for breast cancer (and the evidence is inconclusive for breast cancer mortality). Women, clinicians and policy makers should consider these findings carefully when they decide whether or not to attend or support screening programs.