Overdiagnosis of invasive breast cancer due to mammography screening: Results from the Norwegian screening program
Annals of Internal Medicine, 04/16/2012

Kalager M et al. – Mammography screening entails a substantial amount of overdiagnosis.

• Comparison of invasive breast cancer incidence with and without screening.
• A nationwide mammography screening program in Norway (inviting women aged 50 to 69 years), gradually implemented from 1996 to 2005.
• The percentage of overdiagnosis was calculated by accounting for the expected decrease in incidence following cessation of screening after age 69 years (approach 1) and by comparing incidence in the current screening group with incidence among women 2 and 5 years older in the historical screening groups, accounting for average lead time (approach 2).

• A total of 39 888 patients with invasive breast cancer were included, 7793 of whom were diagnosed after the screening program started.
• The estimated rate of overdiagnosis attributable to the program was 18% to 25% (P < 0.001) for approach 1 and 15% to 20% (P < 0.001) for approach 2.
• Thus, 15% to 25% of cases of cancer are overdiagnosed, translating to 6 to 10 women overdiagnosed for every 2500 women invited.