Breast cancer screening tied to overdiagnosis
WOMEN ‘MISINFORMED’
When they followed women to see whether the number of cancers in the no-screening group caught up with the number spotted during mammograms, the researchers found that it didn’t.
From the difference in cancer rates between women who were screened and those who weren’t, they concluded that 15 to 25 percent of the cancers caught by screening wouldn’t have become symptomatic or life-threatening.
They calculated that for every 2,500 women invited to get screened over ten years, there would be 20 cases of life-threatening breast cancer caught and one breast cancer death prevented. Among that same group of women, between six and 10 would be diagnosed and treated for a tumor that never would have caused them any problems.
One in three women who is told she has breast cancer after screening is being diagnosed and treated unnecessarily, scientists say today.
Not all breast cancers are potential killers, say researchers in a paper in today’s British Medical Journal. Some are inconsequential. If they were not picked up, women would not know they had them. But because they are detected through breast cancer screening, women usually undergo surgery and chemotherapy which are traumatic and potentially harmful.
The Nordic Cochrane Centre group, which did the research, has identified over-diagnosis of breast cancer in the past from the original trials carried out before mammography screening was widely introduced. But in today’s paper, it calculates the extent of that over-diagnosis (detecting harmless cancers) in real populations where screening is offered in the UK, Canada, Australia, Sweden and Norway.
It is no longer contested that screening leads to over-diagnosis, according to an editorial published by the BMJ. “The question is no longer whether, but how often, it occurs,” writes Gilbert Welch, professor of medicine at the Dartmouth Institute for Health Policy and Clinical Practice in the US. “The NHS recently scrapped its leaflet inviting women to undergo mammography in response to criticisms that it failed to mention the major harm of screening – over-diagnosis.”
Some cancers grow so slowly that the person eventually dies of something else, while others are dormant or even regress, he says.
“Because doctors don’t know which patients are over-diagnosed, we tend to treat them all. Over-diagnosis therefore results in unnecessary treatment.
With the advent of widespread efforts to diagnose cancer earlier, over-diagnosis has become an increasingly vexing problem.”
The findings don’t mean that mammography is never worth it, Kalager said, and there’s evidence it does save some lives.
Dongfeng Wu, who models cancer screening at the University of Louisville, said that her own research has suggested the rate of overdiagnosis is generally lower, at about six to nine percent instead of up to 25 percent.
That’s based on estimates of how accurate mammography is, as well as how long it typically takes breast tumors to become symptomatic.
Even with a lower chance of unnecessary diagnosis and treatment, some women may choose to opt out of mammography, said Wu, who wasn’t involved in the new study.
“If a woman thinks she has a higher risk because of her family - close relatives have breast cancer - she might want to (get screened) regularly,” Wu told Reuters Health.
“Or if she considers herself a low-risk person, she might just do it every few years, or she might just skip it.”
The researchers agreed that once women have all the relevant information, they should be allowed to make their own decision on screening without judgment.
“Women are told about the benefits, but not about the harms,” Kalager said. “They are really misinformed.”