by Jared Reed  

Overdiagnosis of breast cancer by screening mammography is greater than previously thought, Australian cancer researchers say.  

Women need to be made aware of the possibility that their positive cancer result might be part of the 42% overdiagnosis rate found in a new study from Sydney University and the Sydney Cancer Centre, the authors say.  

The study of NSW women compared the observed and expected rates of breast cancer in women of screening age groups of 50-69 years of age. Unlike previous research, it took into account changes in breast cancer risk factors as well as lead time, and also looked at cancer rates in unscreened age groups and all women prior to screening implementation.  

The authors say their estimates of overdiagnosis of 30-42% are higher than previous  studies of screening, which suggested overdiagnosis rates of  less than 30%.  

But they say their estimates are robust and probably conservative because they over-adjusted for risk factors and participation rates.  

“It is important that practitioners, policy makers and women attending for, or considering, screening are aware of the potential extent of breast cancer overdiagnosis and consequent overtreatment,” they write in Cancer Causes Control (online 6 November 2009).   

“More research is needed into ways of presenting information, including on overdiagnosis, to help women make an informed choice about whether to participate in screening.”