When Do Changes in Cancer Survival Mean Progress? The Insight From Population Incidence and Mortality

Changes in cancer survival are increasingly being used as a measure to track progress against cancer and appear frequently in medical journal articles, news media reports, and statements from policymakers and advocacy groups. For example, a recent medical journal article argued that higher cancer costs in the United States versus Europe are “worth it” based on increased survival rates (1). And the Komen Foundation’s breast cancer awareness campaign has repeatedly used higher breast cancer survival for screened versus unscreened women as a central argument for why women should undergo mammography (2). Survival statistics are popular because they seem intuitively obvious: increasing survival sounds like good news, decreasing survival sounds like bad news. But the story is more complicated. Improved survival represents progress when it is accompanied by a decreased burden of disease: fewer people being diagnosed or dying from cancer. But improved survival can also occur even when disease burden is increased. Survival is the proportion of cancer patients who are alive at a given time after diagnosis, and in fact it is a complex measure which can be affected by several biases. These biases are introduced by early detection, typically from screening or incidental detection with advanced imaging. Early detection inflates survival by moving back the time of diagnosis (lead time bias), by identifying relatively slow growing, good prognosis cancers (length bias), or by finding cancers never destined to progress at all or which progress so slowly that the person dies of other causes (overdiagnosis). These biases explain how survival can increase even if no deaths are delayed or prevented. Unfortunately, it is impossible to disentangle the effects of early diagnosis solely based on improved survival in population-based cancer surveillance data because cancer registries do not routinely collect the mode of diagnosis (ie, symptomatic, screened, or incidental). To understand whether improved survival represents true progress, it needs to be interpreted in the context of cancer burden: How many people are diagnosed with cancer (incidence) and how many die from it (mortality). Building on the work of Dickman and Adami (3), we analyzed trends in cancer survival for major cancer sites in the United States, together with incidence and mortality. We also illustrate survival and incidence trends by stage at diagnosis. For mortality we provide an example of how incidence-based mortality (IBM) (4) by stage at diagnosis, which is obtained from deaths that occurred from cases reported to the cancer registry, can add in the interpretation of the trends.