Bigger studies are needed to tell whether cancer screening really saves lives, according to a new analysis.
While cancer screening may be linked to fewer deaths from tumors, finding cancers doesn't necessarily save lives when fatalities from all causes are taken into account, the authors point out.
Studies to date may have included too few people to detect slight overall mortality benefits associated with screening, two doctors and a health care journalist argue in The BMJ. It’s also possible that any reduction in cancer deaths due to screening may be offset by fatalities connected to harmful effects of the tests themselves or of unnecessary treatments.
“It is clearly the case that some deaths unrelated to cancer are due to screening, whether from complications of procedures or treatment of cancer,” said lead study author Dr. Vinay Prasad of Oregon Health and Science University.
Many cancers are also over diagnosed, Prasad added by email. This means that in some instances, screening detects abnormal cells that would never have progressed to cause symptoms or complications, or not before the person died of old age or some other cause.
“Yet because of screening, a person may undergo surgery, radiation, chemotherapy and more to treat it – all those treatments have side effects,” Prasad said.
Take stool testing for colorectal cancer. One study found 128 cancer deaths among every 10,000 people who got screened, compared to 192 cancer deaths among every 10,000 individuals who didn’t get screened.
But when researchers looked at deaths from all causes, there wasn’t a meaningful difference between the two groups. To detect any decrease in overall deaths in either group, the authors note, the study would have needed to be five times as large as it was.
Other studies have also linked screening to a slight increase in deaths unrelated to colon cancer, the authors add.
These so-called “off-target” deaths are particularly likely with screening tests that produce what’s known as false positive results, when follow-up evaluations find that people don’t actually have cancer.
False positive screening results contribute to more than one million prostate biopsies a year, the authors note. Down the line, procedures done as a result of these inaccurate results can lead to incontinence, erectile dysfunction, hospital admission and deaths.
Mammograms, too, may be problematic, they write, citing Swiss data showing that these tests avert just one breast cancer death for every 1,000 women screened.
“There used to be ads saying if a woman hadn’t had a mammogram, she needed more than her breasts examined,” Prasad said. “The fact that the medical profession promoted screening so strongly, when it was always a balancing act, when it was always a personal choice, is really shameful.”
The strongest evidence that testing for cancer may save lives comes from a lung cancer study involving more than 53,000 heavy smokers. Half of the participants got standard chest X-rays and the other half received more sensitive CT scans. The CT group showed a 20 percent reduction in the risk of lung cancer deaths and a 6.7 percent decline in overall mortality compared to the X-ray group.
But the absolute reduction in deaths from any cause among those screened was less than half a percent and may have been due to chance, the authors conclude.
To know if cancer screening truly saves lives, statistically robust studies based on millions of people are needed, they write, conceding that it would be expensive, but no more so than supporting mass population screening programs with unproven benefits.
And without conclusive evidence that screening saves lives, doctors have an obligation to clearly outline all the potential risks and benefits of screening to patients so they can make an informed decision about whether it makes sense in their particular situation, Dr. Gerd Gigerenzer, director of the Max Planck Institute for Human Development in Berlin, writes in an accompanying editorial.
Some patients who have the facts and weigh the pros and cons of screening may indeed benefit, but that doesn’t mean doctors should overstate the value of tests and encourage patients to go this route all of the time, Gigerenzer writes.
“The take-home message is after decades of research we have not found clear evidence that screening saves lives, but clear evidence that screening harms many,” Gigerenzer added by email.