Around a third of screening-detected breast cancers in US women in 2008 may have been overdiagnosed, suggests research published in The New England Journal of Medicine.
US Surveillance, Epidemiology, and End Results data were used to determine the incidence of early- and late-stage breast cancer among women aged 40 years and older from the introduction of screening mammography in 1976 to 2008.
The number of cases of ductal carcinoma in situ and localized disease detected each year rose from 112 to 234 cases per 100,000 women, but the detection of regional and distant disease over the same period fell from just 102 to 94 cases per 100,000 women.
The researchers explain that, assuming a constant disease burden, just eight of the 122 additional early-stage cancers detected would have been expected to progress to advanced disease.
"Although it is not certain which women have been affected, the imbalance suggests that there is substantial overdiagnosis, accounting for nearly a third of all newly diagnosed breast cancers, and that screening is having, at best, only a small effect on the rate of death from breast cancer," say Archie Bleyer (St Charles Health System, Central Oregon, USA) and H Gilbert Welch (Dartmouth Institute for Health Policy and Clinical Practice, Hanover, New Hampshire, USA).
After adjusting for breast cancer trends in women aged less than 40 years, and the risk associated with hormone-replacement therapy, the researchers estimate that over the past 30 years 1.3 million US women have been diagnosed with breast cancer that would not have led to clinical disease.
"We estimated that in 2008, breast cancer was overdiagnosed in more than 70,000 women; this accounted for 31% of all breast cancers diagnosed," they write.
Bleyer and Welch therefore recommend: "Proponents of screening should provide women with data from a randomized screening trial that reflects improvements in current therapy and includes strategies to mitigate overdiagnosis in the intervention group."
The study complements research reported previously by medwireNews[w1] indicating that for each woman in the UK whose life is saved with treatment for mammography-detected breast cancer, three patients will receive unnecessary treatment.
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