Breast cancer: conflating case fatality with mortality

While we agree with Taylor and colleagues that the prognosis for women with breast cancer has improved, we are troubled by their use of the term mortality.1 Mortality is conventionally measured in a population at risk of developing a disease, not among those diagnosed with the disease. Case fatality is the conventional term for what was measured in this paper.2The problem is not simply one of semantics: improvements in case based measures—rising survival and declining case fatality—exaggerate the progress being made.3 The problem is in the denominator, which is sensitive to who gets diagnosed with early stage breast cancer and when. That changes over time. More widespread mammography screening and increasingly sensitive diagnostic practices will shift the spectrum of breast cancer towards more slowly growing, less aggressive forms—including breast cancers not destined to progress.4 Even if there is no change in population based mortality, survival will reliably rise and case…
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