Overdiagnosis in breast cancer screening: the importance of length of observation period and lead time.

Stephen W Duffy,Dharmishta Parmar

doi:10.1186/bcr3427

Abstract

BackgroundOverdiagnosis 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.MethodsWe 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.ResultsComparison 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.ConclusionStudies using shorter observation periods will overestimate overdiagnosis by inclusion of cancers diagnosed early due to lead time among the nominally overdiagnosed tumours.

Highlights

The issue of overdiagnosis in breast cancer screening is a topic of much interest and controversy [1,2,3]
In year 10 at ages 70 to 79, there would be 5,452 cancers diagnosed without screening but only 3,268 with screening. This shift in age and time of diagnosis means that if incidence is increasing with age, with time, or with both, the observed excess due to lead time will be larger
The incidences separate dramatically, in the first 10 years, and the disparity remains roughly constant over the following 10 years; the two cumulative graphs come together at 30 years. This demonstrates that the only phenomenon generated by the screening in this scenario is lead time

Summary

Introduction

The issue of overdiagnosis in breast cancer screening is a topic of much interest and controversy [1,2,3]. Overdiagnosis can be estimated by comparison of incidence in a randomized trial of screening, but this would require that the control group was never screened and that both intervention and control groups were followed up to expiry or for more than at least two decades. Overdiagnosis rates are usually estimated on the basis of changes in breast cancer incidence following the introduction of screening services in a population setting [1,2,3,5,6]. 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

Results

Conclusion