There is insufficient evidence to claim cancer screening saves lives, argue experts in a report published in The BMJ, who call for future studies to assess the impact of cancer screening on overall mortality rather than disease-specific mortality alone.
Numerous studies have suggested cancer screening can reduce disease-specific mortality risk. A study reported by Medical News Today last month, for example, claimed ovarian cancer screening can reduce long-term mortality from the disease by 20%.
But what about the effect of cancer screening on overall mortality?
According to Vinay Prasad, assistant professor at Oregon Health & Science University, and colleagues, despite most studies finding a reduction in disease-specific mortality with cancer screening, few have shown reductions in overall mortality, and some have even found an increase in overall mortality.
Furthermore, the authors note that in cases where cancer screening was associated with falls in both disease-specific and overall mortality, the effect was still stronger for disease-specific mortality.
Screening studies ‘underpowered’ to identify benefits to overall mortality
In their report, Prasad and colleagues suggest two key reasons why studies have identified a significantly greater reduction in disease-specific mortality than overall mortality.
They say studies may be “underpowered” to identify small benefits in overall mortality, explaining that such studies fuel assumption and uncertainty about benefits rather than a true assessment of the scientific evidence.
Additionally, the team suggests any reduction in disease-specific mortality could be offset by deaths as a result of the negative effects of cancer screening. “Such ‘off-target deaths’ are particularly likely among screening tests associated with false-positive results, overdiagnosis of non-harmful cancers, and detection of incidental findings,” they explain.
The authors use prostate-specific antigen (PSA) testing as an example, noting that the screening method – which leads to more than 1 million prostate biopsies annually – often leads to false-positive results.
“The overall effect of cancer screening on mortality is more complex than a disease-specific endpoint can capture, owing to the harms of further testing, overdiagnosis, and overtreatment,” note the authors
General public have an ‘inflated sense’ of cancer screening benefits
But despite these shortfalls in scientific research, Prasad and colleagues say data has indicated that the general public have an “inflated sense” of the benefits of cancer screening, but they are less aware of the harms such screening may cause.
The authors cite a study that found 68% of women believed mammography – a screening method for breast cancer – would reduce their risk of developing breast cancer, 62% believed the screening halves breast cancer rates, while 75% believed mammography would prevent 10 deaths from breast cancer in every 1,000 women.
However, they point to a Cochrane review of mammography that found no reduction in breast cancer deaths “when adequately randomized trials were analyzed.”
So, what is driving this “inflated sense” of cancer screening benefits? The authors believe supporters of cancer screening have focused on promoting the benefits of screening rather than harms, and they suggest that some screening advocates even engage in fear-mongering.
“But as long as we are unsure of the mortality benefits of screening,” say the authors, “we cannot provide people with the information they need to make an informed choice. We must be honest about this uncertainty.”
They say individuals should be informed that there is no clear evidence on the mortality benefits of cancer screening, adding:
“We encourage health care providers to be frank about the limitations of screening – the harms of screening are certain, but the benefits in overall mortality are not. Declining screening may be a reasonable and prudent choice for many people.”
The authors state that financial resources, public perception of cancer screening and political will are factors that are holding back trials that are solid enough to effectively assess the impact of cancer screening on overall mortality.
While they admit that breaking down these barriers will “take time and effort,” they believe investment into such trials is worthwhile, particularly when compared with the ongoing cost of supporting cancer screening campaigns without knowing whether screening is really beneficial to the general public.
“We call for higher standards of evidence, not to satisfy an esoteric standard, but to enable rational, shared decision-making between doctors and patients,” the authors conclude.
Screening communication ‘should move away from dodgy persuasion’
In an accompanying editorial, Gerd Gigerenzer, of the Max Planck Institute for Human Development in Berlin, Germany, agrees that the benefits of cancer screening are overstated but the harms are understated.
However, he says that rather than investing in cancer screening “megatrials” that only hold a small chance of identifying a limited reduction in overall mortality, it is better to invest in clear-cut information at the outset.
Gigerenzer adds that even if the benefits of cancer screening on overall mortality cannot be established, there are other ways in which health care providers can help patients to make informed decisions.
For example, he says fact boxes could be used to inform women of disease-specific mortality rates associated with mammography – something he says is already used at Max Planck, based on the results of a Cochrane review.
“The harms are specified numerically so that an informed decision about screening is possible,” he explains. “Every article and pamphlet should provide a fact box summary to facilitate informed decisions.”
“It is time to change communication about cancer screening from dodgy persuasion into something straightforward,” Gigerenzer concludes.
In October of last year, MNT reported on new breast cancer screening guidelines set by the American Cancer Society, which state that women should begin annual screening from the age of 40-54 and be given the option to continue screening after the age of 55.
Written by Honor Whiteman