Overdiagnosis of breast cancer

Overdiagnosis of invasive breast cancer could be as high as 42 per cent, a paper published by researchers at the University of Sydney’s School of Public Health suggests.

The study shows the reduced mortality rate resulting from increased mammography screening comes with a flipside: there is 30 to 42 per cent excess of women diagnosed with invasive breast cancer, over that expected, who undergo unnecessary treatment. This translates to about 23 to 29 per cent of all breast cancers diagnosed in NSW as being overdiagnosed and over treated.

Other studies have looked into overdiagnosis of breast cancer before but our findings are unique," says Associate Professor Alex Barratt, one of the paper’s authors. "They take into account risk factors such as hormone replacement therapy and obesity, both of which have increased the prevalence of breast cancer since about the same time as publicly-funded mammography screening programmes were introduced.

"Even when these risks are accounted for, we still find breast cancer overdiagnosis is very high. Overdiagnosis means many women are needlessly having surgery or other treatment at great personal cost."

Associate Professor Barratt says the paper – "Estimates of overdiagnosis of invasive breast cancer associated with screening mammography" – indicates the pressing need to develop tests to predict which screening-detected cancers are likely to progress.

"The problem is right now we just don’t know which way a cancer detected through mammography is going to go," she says. "We know there is overdiagnosis across the population but we can’t pinpoint which women are overdiagnosed.

There is also a need for better information for women and doctors, quantifying the benefits and the harms of mammography screening. "We have that information now and it would be good to provide it to people in an easy to use form, such as a patient decision aid."

"Estimates of overdiagnosis of invasive breast cancer associated with screening mammography" was published in the latest issue of the Cancer Causes Control journal. Its calculations were based on the incidence of invasive breast cancer in New South Wales but Associate Professor Barratt says the findings apply across Australia and similar countries.

The study did not investigate overdiagnosis of pre-invasive ductal carcinoma in situ (DCIS), which accounts for about 18 per cent of breast cancers diagnosed by NSW’s publicly funded screening programme.

(Source: University of Sydney: Cancer Causes Control: November 2009)