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A change in cancer stage distribution, also known as a ‘stage shift’, is a common outcome measure in cancer epidemiology. A favourable stage shift—an increased proportion of early-stage cancer—is often the goal of early cancer detection efforts. An unfavourable stage shift—an increased proportion of late-stage cancer—is typically seen as evidence of the harm of diminished screening. Shifting cancer stage distributions, however, can be deceptive.
Consider the aspirational goal of the UK’s National Health Service (NHS): to increase the proportion of all cancers diagnosed at an early stage from approximately 50%–75%.1 The hope is that, by finding more people with early-stage cancer, the number presenting with late-stage cancer will fall. But even if only more early-stage cancers are detected, with no change in the number of late-stage cancers—a signature highly suggestive of cancer overdiagnosis—the proportion of all cancers diagnosed at an early stage will nonetheless rise.
The problem is in the measure. Stage distribution is a proportion; thus, changes can be driven by changes in the numerator, denominator or both. That makes changes difficult to interpret.
Alternatively, consider the interest in measuring the effect of COVID-19-related reductions to cancer screening. Here, the concern is whether diminished screening is associated with cancers being diagnosed later, leading to an increased proportion of late-stage cancer. But, of course the stage distribution will appear to be less favourable and a higher proportion will be late-stage cancer—not necessarily …
Contributors JO and HGW conceived the idea, drafted the first version of the manuscript and equally contributed to revisions. JO is the guarantor.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests None declared.
Provenance and peer review Not commissioned; externally peer reviewed.