Karsten J. Jørgensen, MD
Potential Conflicts of Interest: None disclosed.
Jørgensen K.; Overdiagnosis of Invasive Breast Cancer due to Mammography Screening. Ann Intern Med. 2012;157:219-220. doi: 10.7326/0003-4819-157-3-201208070-00022
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Published: Ann Intern Med. 2012;157(3):219-220.
TO THE EDITOR:
We estimated 37% overdiagnosis of invasive breast cancer in Norway and 52% when we included cases of ductal carcinoma in situ (DCIS) (1). This means that 1 in 3 cases of breast cancer detected in the screened age group is overdiagnosed. We estimated 33% overdiagnosis in Denmark (2), which has the longest period worldwide that allows a contemporaneous comparison between screened and nonscreened areas.
There are 3 main reasons for the difference between our estimates for Norway and the estimated 18% to 25% overdiagnosis of invasive breast cancer in Kalager and colleagues' study (3). We prefer the analysis that relies on observed numbers rather than on an assumed lead time. First, we included DCIS in our analysis; second, we did not include the nonscreened group aged 70 to 79 years; and third, we define overdiagnosis in a slightly different way. Kalager and coworkers state that including cases of DCIS requires special considerations that they could not honor, but their results indicate that overdiagnosis of DCIS is probably more common than we assumed, as we counted half the identified DCIS lesions as overdiagnosed (2). If DCIS lesions identified at screening were not a product of overdiagnosis, removal should lower the frequency of invasive cancer in the future. This did not happen in Norway (3) or elsewhere (4, 5). Incidence of early-stage, invasive breast cancer skyrockets with screening (1,), and rates of late-stage, invasive breast cancer have not been reduced (4). Given that DCIS constitutes between 10% and 25% of breast cancer cases in a screened population, we believe it is essential to include a proportion of them in an estimate of overdiagnosis if we are to make a meaningful comparison of the benefits and harms of screening. The largely nonscreened group aged 70 to 79 years was included to take into account a compensatory reduction in breast cancer incidence in previously screened women (lead-time bias). We found a decline in this age group in the screened areas in Norway in the relevant period, but also a similar reduction in the nonscreened areas (2). We conservatively chose to compensate for the decline in our estimate of overdiagnosis, subtracting it from the observed increase in incidence in the screened group aged 50 to 69 years. Simply including the 70- to 79-year age group substantially dilutes the true level of overdiagnosis.
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