33% of breast cancers detected may be harmless

ONE in three breast cancers detected by screening may actually be harmless.

Data from Britain, Canada, Australia, Sweden and Norway suggests some women undergo unnecessary treatment for cancers unlikely to kill them or spread.

Some cancers grow so slowly the patient dies of other causes first, or the cancer remains dormant or regresses.

Researchers from the Nordic Cochrane Centre in Denmark said cancer screening programmes could lead to “over-diagnosis”.

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 over-diagnosis and can only be harmful to those who experience it.

“As it is not possible to distinguish between lethal and harmless cancers, all detected cancers are treated.

“Over-diagnosis and over-treatment are therefore inevitable.”

The authors pointed to autopsy data which has shown that about 37% of women aged 40 to 54 who died from causes other than breast cancer, had lesions of invasive or non-invasive 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 between 1971 and 1999.

The experts estimate the level of over-diagnosis to be between 46% and 59%, with a total over-diagnosis of 52%.

“We estimated 52% over-diagnosis of breast cancer in a population offered organised mammography screening — that is, one in three breast cancers is over-diagnosed,” they said.