Does screening mammography lead to overdiagnosis of invasive breast cancer?

www.aafp.org/afp Volume 87, Number 6 ◆ March 15, 2013 What does this article say? Jill: This is a prospective historical cohort study looking at the rate of overdiagnosis of invasive breast cancer between 1986 and 2005 in 39,888 Norwegian women 50 to 69 years of age. Norway piloted mammographic screening in four counties that represented 40 percent of the eligible population in 1996 before implementing the program nationwide. Overdiagnosis was said to occur if the breast cancer detected was not likely to become clinically apparent or be the cause of death. This was calculated as the number of incident cases of invasive breast cancer in screened versus unscreened women based on a national registry. Researchers also compared historical and current incidence rate ratios in counties with and without screening programs to account for temporal changes in incidence. They evaluated overdiagnosis using two statistical approaches: (1) by accounting for the expected decrease in incident cases after screening cessation at 69 years of age, and (2) by comparing incidence in the screening group with that among women two and five years older in the historical screening group (to account for lead-time bias). This assumes that breast cancer, if present, will manifest within five years. The risk of overdiagnosis of invasive breast cancer from mammographic screening was 18 to 25 percent using the first approach and 15 to 20 percent using the second approach. Put another way, by screening 2,500 women 50 to 69 years of age biennially with mammography, 20 women were diagnosed with invasive breast cancer, one death was prevented, and six to 10 additional women were overdiagnosed. In addition, the incidence of advanced breast cancer was similar in both the screened and unscreened groups.