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Up to one-fourth of Norwegian women overdiagnosed due to mammograms
The rate of overdiagnosis of breast cancer was 15% to 25%, depending on screening approach, which indicated that six to 10 women are overdiagnosed for every 2,500 women invited, according to results from a nationwide study of a Norwegian mammography program.
The screening program began in 1996, and by 2005, 77% of women aged 50 to 69 years invited to participate had enrolled. Researchers compared concomitant incidence of invasive breast cancer in counties where the screening program was implemented against cancer incidence in counties where the program was not yet implemented. To adjust for changes in temporal trends in breast cancer incidence, researchers also analyzed incidence rates during the preceding decade.
Researchers designated counties that started the screening program in 1996 — 40% of the total Norwegian population — as “region 1.” Follow-up for that area is 10 years. Researchers then calculated percentage of overdiagnosis by accounting for the expected decrease in incidence after cessation of screening after age 69 years (approach 1), and by comparing incidence in the current screening group with incidence among women in the historical screening groups who were 2 and 5 years older, accounting for average lead time (approach 2).
A total of 39,888 women diagnosed with invasive breast cancer from 1986 to 2005 were included in this study. That number includes 27,238 women aged 50 to 79 years and 7,793 who were diagnosed after the screening program began.
In approach 1, researchers estimated the percentage of overdiagnosis was 18% in region I and 25% in the whole country (P<.001). Overdiagnosis was most common among women aged 65 to 69 years.
For approach 2, with an average lead time of 2 years, there was a 59% increase in incidence in the screening group compared with the historical screening group of women aged 52 to 71 years. Estimated rate of overdiagnosis attributable to the mammography screening program was 15% to 20% (P<.001).