One in three breast cancers detected by mammogram screening may actually be harmless, a study has suggested.

一项调查显示，通过乳房X线照片可以分析出三分之一的乳腺癌是无害的。

A mammogram is used to detect breast lesions

Data from five countries, including the UK, suggest some women may have had unnecessary treatment for cancers that were unlikely to kill them or spread.

As it is not possible to distinguish between lethal¹（致命的） and harmless cancers, all are treated.

But advocates（提倡者，拥护者） of screening insist it is a vital tool for early detection of cancerous cells.

Screening programmes

Researchers from the Nordic Cochrane Centre in Denmark said their results showed cancer screening programmes could lead to "overdiagnosis".

Writing in the British Medical Journal (BMJ), they said: "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."

They looked at a range of statistics from five countries which had implemented²（应用的） screening programmes, including data for England and Wales from between 1971 and 1999.

The findings seem to confirm research published by the same team earlier this year.

Dr Sarah Cant³, from Breakthrough Breast Cancer, said she hoped the research would not discourage women from attending screening.

"Unfortunately, it is currently not possible to predict which cancers found through screening will develop aggressively and which will grow very slowly," she said.

"Based on all the current evidence, we believe the benefits of detecting breast cancer early still outweigh⁴ the risks."

She added that women needed to be given clear information about breast screening and it was important to remember that "while survival rates have increased greatly in recent years, just under 12,000 women still die from this disease each year in the UK."