Abstract
BackgroundEstimating overdiagnosis in cancer screening is complicated. Using observational data, estimation of the expected incidence in the screening period and taking account of lead time are two major problems.MethodsUsing data from the Cancer Registry of Norway and the Norwegian Breast Cancer Screening Programme, we estimated incidence trends, using age-specific trends by year in the pre-screening period (1985–95). We also estimated sojourn time and sensitivity using interval cancers only. Thus, lead time estimates were uncontaminated by overdiagnosed cases. Finally, we derived estimates of overdiagnosis separately for all cancers, and for invasive cancers only, correcting for lead time, using two different methods.ResultsOur results indicate that overdiagnosis of all cancers, invasive and in situ, constituted 15–17% of all screen-detected cancers in 1996–2009. For invasive cancers only, the corresponding figures were -2 to 7% in the same period, suggesting that a substantial proportion of the overdiagnosis in the Norwegian Programme was due to ductal carcinoma in situ.ConclusionUsing short-term trends, instead of long, prior to screening was more effective in predicting incidence in the screening epoch. In addition, sojourn time estimation using symptomatic cancers only avoids over-correction for lead time and consequently underestimation of overdiagnosis. Longer follow-up will provide more precise estimates of overdiagnosis.
Highlights
Overdiagnosis in the context of cancer screening is the diagnosis, as a result of screening, of cancer which would not have been diagnosed in the lifetime of the host if screening had not taken place.[1]
In the absence of trial data, a way to estimate overdiagnosis is from trends in observational data on national or regional incidence of breast cancer, in conjunction with the time of introduction of screening.[2,3,4,5]
Our first method calculated the total observed cancers in the screening period and age range, and subtracted from these the total expected from pre-screening trends, the deficit observed above the screening age range and the number of screendetected cancers which would have been expected to arise symptomatically only after the period of observation, but the diagnosis of which was brought forward to our period of observation by lead time
Summary
Overdiagnosis in the context of cancer screening is the diagnosis, as a result of screening, of cancer which would not have been diagnosed in the lifetime of the host if screening had not taken place.[1]. Some of the excess will be due to lead time, the diagnosis as a result of screening of cancers which would otherwise have been diagnosed symptomatically some years later. If the sojourn time is distributed as exponential with mean 1/, which fits breast cancer data reasonably well,[13] this becomes This differs slightly from the more complex formulae in Duffy et al.[14] as the latter apply to a general time t, not necessarily one year, and assume that tumours can remain in the preclinical detectable phase for several rounds of screening and be missed at each successive round. We used the sojourn time estimates to further subtract from the excess any screen-detected cancers expected to be symptomatically diagnosed after the period of observation (i.e. after 2009). The proportions of screen-detected cancers diagnosed in 2001–5 and 2006–9 would be eÀ6:5 and eÀ2
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