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Breast cancer screening causes more damage than previously thought by Sarah Boseley

Around 4,000 women have unnecessary treatment for a disease that will never threaten their health, though tests should continue.

Breast cancer screening causes more harm than has previously been recognised, even though it saves lives, according to an independent review set up following years of scientific controversy surrounding the NHS programme.

Around 1,300 lives are saved every year by mammography, which women are invited to undergo between the ages of 50 to 70, said the review, which recommends that screenings should continue.

But 4,000 women will undergo unnecessary treatment, including surgery, radiotherapy and chemotherapy, for a cancer they would not otherwise have known about and which would have done them no harm in their lifetime. Some breast cancers are so tiny and slow growing that they would never be a threat to a woman's health, the review says.

The government is embarking on an immediate revision of the leaflet which invites women for screening, said national cancer director Sir Mike Richards, so that women can weigh up the benefits against potential harm and make their own decision as to whether to be screened.

While he welcomed the review panel's support for screening, he added: "The key thing is that we communicate this new information to women so they can make an informed choice for themselves.

"NHS Cancer Screening Programmes have already asked independent academics to develop new materials to give the facts in a clear, unbiased way. I hope to see them in use in the next few months. If any woman has concerns about breast screening she should talk to her GP or health professional."

The independent review panel was set up by the government and the charity Cancer Research UK under the chairmanship of Sir Michael Marmot and comprised scientists who had never published on breast screening before, in order to establish their impartiality on an issue that has provoked impassioned argument between epidemiologists.

The leading critic of screening is Professor Peter Gøtzsche, director of the Nordic Cochrane Centre, whose team has spent years analysing the data from the trials carried out in several countries prior to the setup of national screening programmes – as well as more recent studies. Gøtzsche says the harm of screening outweighs the benefits.

Marmot's team also reviewed data from original breast screening trials, many of which took place in Scandinavia. They acknowledged that they had limitations because they mostly date from the 1980s or earlier – the NHS programme was set up in 1988. But they dismissed other criticisms, such as that some of the women had died of other diseases and not breast cancer.

The review judged that screening reduces the risk of dying from breast cancer by 20%. It prevented 43 deaths for every 10,000 women invited to be screened, or one death per 235 women invited. Among those 10,000 women, 681 cancers would be diagnosed and 129 of those would be over-diagnosed – the mammogram would have picked up an otherwise undetectable tumour which the woman would never have known about. As with many prostate cancers in men, it would grow so slowly that without treatment she would die with it, rather than of it.

"Our best estimate is around 4,000 cancers a year which is about 19%," said Professor David Cameron from Edinburgh University, who presented the findings which are published today in the Lancet medical journal.

"Breast cancer screening should continue, but we felt that the information given to women needs to be reviewed with our findings taken into account, so women are given an accurate picture of the benefits and harms."

The current NHS leaflet mentions misdiagnosis almost in passing. "Screening can find cancers which are treated but which may not otherwise have been found during your lifetime," it tells women.

Marmot said the information must now be available for women to make a choice. "Clear communication of these harms and benefits to women is essential, and the core of how a modern health system should function," he said.