January 6, 2016

expert reaction to cancer screening and mortality analysis

The impact of cancer screening programmes on mortality is the subject of a piece in The BMJ, in which the authors argue that overall mortality rather than disease-specific mortality should be the main outcome measured.

Prof. Paul Pharoah, Professor of Cancer Epidemiology, University of Cambridge, said:

“Vinery Prasad and colleagues highlight the lack of evidence for an often quoted “fact” that screening “saves lives”. Many trials of screening for different diseases have been done: some have shown a reduction in death rates from the disease being screened for; a handful have shown a reduction in overall mortality; and some have found an increase in overall mortality associated with screening. Screening has not been shown to save lives. They discuss the possible reasons for this apparent discrepancy. The reasons are complex, but one key reason is that screening often results in over diagnosis – detection of disease that would not have been detected in the person’s lifetime in the absence of screening – and the treatment of over diagnosed disease may be harmful.

“There was a very good paper reviewing the results of 48 screening trails and nine meta-analyses of trials published last year that is mentioned in this analysis. In the nine meta-analyses with ten interventions evaluated none showed a significant difference in overall mortality. Four had non-significant reductions in risk (relative risk reduction of 1 or 2%), three had no effect at all (relative risk reduction 0%) and three had an increase overall mortality with screening (relative risk increase 1, 3 and 34%). So one cannot say for any specific screening intervention that the best evidence suggests that there have been lives saved.

“Prasad and colleagues point out that carrying out trials that would be large enough to detect a small reduction in overall mortality, if it exists, would be an expensive and formidable challenge. Nevertheless, for some diseases, where the infrastructure for running a trial already exists such as the NHS Breast Screening Programme, a very large trial would be possible at a cost commensurate with the current cost of running the programme. They conclude by calling for policy makers and those that provide screening services to be more frank about the limitations of screening and to recognise that declining screening may be a reasonable choice for many people.”

‘Why cancer screening has never been shown to “save lives”—and what we can do about it” by Prasad et al. published in The BMJ on Wednesday 6th January. 

Declared interests

Prof. Paul Pharoah: I have no conflicts of interest.