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.
Screening
tests are available for a number of cancers, including breast,
cervical, colorectal, lung, prostate and ovarian cancers.
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 mortalityIn
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.
Such results
have been associated with increased risk of hospital admission and
death, and some studies have linked prostate cancer diagnosis to
increased risk of heart attack and suicide.
"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 benefitsBut
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."
