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Screening mammography aims to lower incidence of advanced tumors by detecting small, nonpalpable tumors. Overdiagnosis refers to screen detection of tumors not destined to become clinically manifest, leading to unnecessary treatment. In Denmark, biennial screening mammography was introduced in different regions of the country beginning in 1991, with 20% of women aged 50–69 invited to participate during the first 17 years of the program.
To assess screening's effect on detection of both nonadvanced tumors (≤20 mm at detection) and advanced tumors (>20 mm), investigators identified all Danish women (age range, 35–85) with invasive breast cancer diagnosed from 1980 to 2010 and compared incidence rates (IRs) of advanced versus nonadvanced tumors in screened and unscreened populations. To estimate overdiagnosis, they compared IRs of advanced and nonadvanced tumors before and after screening was initiated and among women who were younger (range, 35–49) and older (range, 70–84) than those who were screened.
While screening increased the IR of nonadvanced tumors, it did not decrease the IR of advanced tumors. Among women aged 50–69 (invasive tumors only), estimated rates of overdiagnosis ranged from 14.7% to 38.6%.
Jørgensen KJ et al. Breast cancer screening in Denmark: A cohort study of tumor size and overdiagnosis. Ann Intern Med 2017 Jan 10; [e-pub]. (http://annals.org/aim/article/2596394/breast-cancer-screening-denmark-cohort-study-tumor-size-overdiagnosis)
Brawley OT.Accepting the existence of breast cancer overdiagnosis. Ann Intern Med 2017 Jan 10; [e-pub]. (http://dx.doi.org/10.7326/M16-2850)
Comment
Because overdiagnosis cannot be directly measured, it can be difficult to grasp as a concept. Important strengths of this Danish study include the long follow-up period after initiation of screening and the contemporaneous assessment of screened and unscreened women, which should help address certain concerns of those skeptical about overdiagnosis. I agree with the authors' conclusion that screening has not achieved the promised reduction in breast cancer mortality. I also agree with the view of the editorialist (Chief Medical Officer, American Cancer Society): Although substantial overdiagnosis rates suggest that the benefits of screening have been overstated — and some “cured” women have in fact been harmed by unneeded treatment — we should not necessarily abandon screening altogether. Indeed, by focusing on higher-risk women, future screening strategies could yield better risk-benefit ratios. Until then, I will continue to recommend screening based on U.S. Preventive Services Task Force guidance: Begin biennial screens at age 50, recognizing that some women favor earlier and more-frequent screens while others prefer less-frequent or no screening.