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. 2013 May 16;15(3):R41.
doi: 10.1186/bcr3427.

Overdiagnosis in Breast Cancer Screening: The Importance of Length of Observation Period and Lead Time

Free PMC article

Overdiagnosis in Breast Cancer Screening: The Importance of Length of Observation Period and Lead Time

Stephen W Duffy et al. Breast Cancer Res. .
Free PMC article

Abstract

Background: Overdiagnosis in breast cancer screening is a controversial topic. One difficulty in estimation of overdiagnosis is the separation of overdiagnosis from lead time that is the advance in the time of diagnosis of cancers, which confers an artificial increase in incidence when a screening programme is introduced.

Methods: We postulated a female population aged 50-79 with a similar age structure and age-specific breast cancer incidence as in England and Wales before the screening programme. We then imposed a two-yearly screening programme; screening women aged 50-69, to run for twenty years, with exponentially distributed lead time with an average of 40 months in screen-detected cancers. We imposed no effect of the screening on incidence other than lead time.

Results: Comparison of age- and time-specific incidence between the screened and unscreened populations showed a major effect of lead time, which could only be adjusted for by follow-up for more than two decades and including ten years after the last screen. From lead time alone, twenty-year observation at ages 50-69 would confer an observed excess incidence of 37%. The excess would only fall below 10% with 25 years or more follow-up. For the excess to be nullified, we would require 30 year follow-up including observation up to 10 years above the upper age limit for screening.

Conclusion: Studies using shorter observation periods will overestimate overdiagnosis by inclusion of cancers diagnosed early due to lead time among the nominally overdiagnosed tumours.

Figures

Figure 1
Figure 1
Expected cumulative incidence over ages 50 to 79 in a cohort of women of age 50 at the start, with and without 2-yearly screening from age 50 to 69.
Figure 2
Figure 2
Expected cumulative incidence over 25 years in a screened group of ages 50 to 54 and in an unscreened group of age 55 to 59 at recruitment at the start of follow-up.

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