"Over diagnosis" a problem in breast cancer 

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About a quarter of Australian women undergoing treatment for breast cancer have a tumour growing so slowly that it could have been safely ignored, research indicates.

A NSW-based study into the introduction of routine breast screening for older women has found the move corresponded with a much higher than anticipated rate of cancer diagnoses.

Dr Stephen Morrell said the rate of new cancers was well above the known background level of breast cancer in the community, even when accounting for rising risk factors like obesity.

He said it pointed to an overly sensitive screening process that resulted in unnecessary and sometimes life-changing treatment, such as mastectomy, for some women whose tumours did not pose a health risk.

"We did some basic calculations to estimate what cancers we expected from the underlying incidence before screening and took into account incidence that should have occurred under screening ... There was a large excess of cancers," said Morrell, from the University of Sydney's school of public health.

"Even taking into account that expected rise and even the rise in risk factors ... there is an excess that is otherwise unexplainable.

"We probably are picking up a lot of indolent cancers; very small ones that would not have progressed in the woman's life."

Morrell estimated that of all breast cancers now diagnosed in NSW, 23 to 29% were "over-diagnosed" and could be safely left alone.

Along with colleagues, Morrell analysed all breast cancer diagnoses added to a NSW-based register from 1972 to 2001.

He said before the introduction of breast screening, the cancer incidence among Australian women aged 50 to 69 years was about 150 cases per 100,000 in the population.

After screening was fully established the incidence jumped to almost 300 cancer diagnoses per 100,000 women and, Morrell added, "that's a big rise".

The problem of over-diagnosis is also an issue for prostate cancer screening in men, because while it is known to lead to the early detection of many problematic cancers it can also prompt unnecessary treatment.

"I think overall mammography is still a net benefit, mortality has declined due to mammography screening, but there is a price to pay and it is a certain number of women who are treated unnecessarily for every life saved," Morrell said.

"The main problem is there is no system for triage, working out which cancer are baddies and which are goodies ... that this one will go ahead and that one won't. At the moment we don't have the technology."

Morrell said women who undergo the screening, and who are diagnosed with cancer, should be given more information about the potential for over diagnosis.

The research is published in the journal Cancer Causes and Control.