Women may be undergoing gruelling cancer treatment needlessly because as many as one in three cases of breast cancer detected by screening prove harmless, a study shows today.

Data from Britain, Canada, Australia, Sweden and Norway suggests that a proportion of women have breast cancer “overdiagnosed” when the disease is unlikely to kill them or spread to other parts of the body.

The study, reported today in the British Medical Journal, comes after 23 leading health specialists criticised the Government’s “unethical” failure to provide women with full facts about the NHS screening programme for all women aged 50 to 70.

In a letter to The Times in February, the specialists said that many women have breast cancer diagnosed even if they have benign tumours and may undergo unnecessary surgery, radiotherapy or chemotherapy as a result.

The latest study, by a team from the Nordic Cochrane Centre in Denmark, says that screening for cancer may lead to earlier detection of lethal cancers but also detects harmless ones that will not cause death or symptoms.

“The detection of such cancers, which would not have been identified clinically in someone’s remaining lifetime, is called overdiagnosis and can only be harmful to those who experience it,” the researchers add.

“As it is not possible to distinguish between lethal and harmless cancers, all detected cancers are treated. Overdiagnosis and overtreatment are therefore inevitable.”

The authors pointed to post-mortem data that has shown that about 37 per cent of women aged 40 to 54 “who died from causes other than breast cancer, had lesions of . . . cancer at autopsy”. About half of these lesions would have been picked up in screening and some would have been treated, they said.

The experts included screening data for England and Wales from 1971 to 1999. They looked at breast cancer trends before and after the introduction of government-funded screening programmes in all countries.

To minimise bias, their data covered at least seven years before and after screening had been implemented in each country.

Julietta Patnick, director of the NHS Cancer Screening Programmes, said that one in eight (12.5 per cent) women who had breast cancer diagnosed through the NHS Breast Screening Programme would not have had the disease detected if they had not gone for screening.

“Screening reduces cancer mortality through early detection, making treatment more likely to be effective. It is estimated that it saves 1,400 lives every year in England.”

She added that the Danish study appeared to be “highly selective in the statistics that it used; it ignores women’s lifestyle changes which have increased breast cancer incidence in England, such as women waiting longer before having their first child and taking HRT”.

Sarah Cant, from Breakthrough Breast Cancer, said she hoped this research would not discourage women from attending screening. Although survival rates had improved greatly in recent years, breast cancer still kills 12,000 women a year.

“Unfortunately, it is not yet possible to predict which cancers found through screening will develop aggressively or grow very slowly. We believe the benefits of early detection still outweigh the risks,” she said.

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