Cancer screening is designed to detect the presence of disease at an early stage, but whether it actually has the desired effect of reducing mortality is up for debate. Clinical writer Rod Tucker investigates.
In medicine, screening is designed to identify the presence of a disease in healthy individuals who are not displaying any of the typical clinical signs or symptoms of a particular condition. Cancer screening, for instance, can therefore be considered to serve a dual purpose: on the one hand it is preventative, but it can also enable the early identification of cancer or precursor lesions and thus having the potential to improve prognosis.
Cancer screening is already well established for breast, prostate, lung, cervical and colorectal cancers, and many of the current tests are very accurate. For example, one novel prostate cancer screening test has an accuracy of 94%.
The fact that cancer screening has the potential to reduce cancer-related mortality and ultimately improve lifespan, has been the main focus in public health messaging. In the past, such messaging has been emotive, employing powerful and persuasive tools such as fear and guilt, engendering a sense of personal responsibility and convincing individuals to be screened.
And this approach has clearly worked. For instance, in a survey of 500 individuals that included women over 40 years of age and men over 50 years, without a history of cancer, the majority (87%) believed that routine cancer screening was almost always a good idea. In addition, 74% felt that finding a cancer early, through methods such as screening, saved lives most or all the time.
But does cancer screening actually extend lifetime? Five-year survival is often presented to the public as evidence of the value of early detection, but one analysis concluded that changes in five-year survival over time bear little relationship to changes in cancer mortality.
A recent meta-analysis attempted to shed further light on the relationship between cancer screening and all-cause mortality, which offered some surprising results.
Cancer screening and all-cause mortality
Most randomised controlled trials of cancer screening focus on disease-specific mortality as the outcome of interest. However, the validity of this outcome is highly dependant on an accurate ascertainment of death, which is not always clear.
This was highlighted in an analysis of 12 published randomised trials of cancer screening for which disease-specific and all-cause mortality data were available. The researchers identified major inconsistencies in both disease-specific and all-cause mortality but concluded that since all-cause mortality is not affected by bias in classifying the cause of death, it should be used as the main outcome of interest for randomised trials looking at cancer screening.
The most recent study to examine of the impact of cancer screening on all-cause mortality was conducted by a team based at the Institute of Health and Society, which is part of the University of Oslo in Sweden. The researchers performed a systematic review and meta-analysis of randomised clinical trials that reported on all-cause mortality to estimate the potential lifetime gained for six commonly used cancer screening tests when compared to no screening.
They focused on mammography screening for breast cancer; colonoscopy, sigmoidoscopy or faecal occult blood testing (FOBT) for colorectal cancer; computed tomography screening for lung cancer in smokers and former smokers; and prostate-specific antigen (PSA) testing for prostate cancer.
The team included trials with 10 to 15 years of follow-up and set the primary outcome of interest as lifetime in the screened group compared to the non-screening group based on all-cause mortality. Using these differences, the researchers calculated the absolute lifetime gained in days.
With the inclusion of just over 2.1 million individuals in the analysis for the six different cancer screening trials, the only screening test with a significant lifetime gain of 110 days (95% CI 0 – 274) was sigmoidoscopy. However, the lower 95% confidence interval extended to zero, so the effect was just significant.
Although there were gains in lifetime accrued for prostate (37 days) and lung (107 days) cancer screening, these increases were not significant.
Risk of death from other causes
Despite these largely negative findings, the researchers were quick to point out that they did not actually favour abandoning screening, merely that the current evidence did not substantiate the claim that common cancer screening tests saved lives by extending lifetime.
Although all-cause mortality has been suggested as a better study endpoint, a problem with using this metric is that the impact of screening on all-cause mortality, depends largely on the level of underlying risk of death from different causes, such as heart disease, within the population being screened.
Consequently, while there are clearly some limitations from focusing solely on cancer-related deaths as an outcome, this may be preferable with the caveat that deaths from other causes are carefully reviewed to identify potential cases of harm.
Nonetheless, a 2015 analysis concluded that currently available screening tests for diseases where death is a common outcome, reductions in disease-specific mortality are uncommon and either very rare or non-existent for all-cause mortality.
The use of all-cause mortality as a primary outcome may be more relevant for the multi-cancer early detection test, especially if deaths from the cancers targeted constitute a high proportion of expected deaths in the population recruited.
Perhaps the most important message from the recent cancer screening analysis is that in their discussions with patients, clinicians should no longer predicate the rationale for screening on the claim that it will both save lives and increase life expectancy. This may not always be the case.
