Overdiagnosis in the Norwegian Breast Cancer Screening Programme Institute of (your Institute)
Research Title Here Dimitrios
1
1 Michalopoulos , Stephen
W.
1 Duffy
Authors
Centre for Cancer Prevention, Queen Mary University of London, UK
Figure 1: Observed and expected incidence rate by age group in the screening period (1996-2009)
Background 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. Using observational data, estimation of the expected incidence in the screening period and taking account of lead time are two major problems researchers have to overcome.
Incidence rate per 100000 women
350 300 250 200 150 100 50 0
Goal: To estimate overdiagnosis using short term trends and lead time estimates derived entirely from interval cancers (not overdiagnosed cases).
Data and Methods Data were collected from the Cancer Registry of Norway and the Norwegian Breast Cancer Screening Programme (NBCSP). The Norwegian Breast Cancer Screening Programme was initiated in November 1995 offering biennial 2-view mammography to women aged 50-69 years. Log-linear trends were estimated per individual calendar year within each 5-year age group from 50-54 to 80-84, using data from the pre-screening period (1985-1995), and projected these in the screening epoch (1996-2009). Sojourn time and sensitivity were estimated using interval cancers only. Estimates of overdiagnosis were derived separately for all cancers, and for invasive cancers only, correcting for lead time, using two different methods.
55-59
65-69
70-74
75-79
80-84
Expected
Excess incidence tended to be highest in the oldest screening age group, 65-69. This is consistent with overdiagnosis being greater at older ages, due to shorter future life expectancy and longer lead times. Overdiagnosis of all cancers, invasive and in situ, constitute 15-17% of screen-detected cancers in 1996 – 2009 (Table 1). For invasive cancers alone, the corresponding figures were 0-7% of screen-detected cancers. Table 1: Estimates of overdiagnosis for all cancers and invasive cancers only using two different methods Overdiagnosis as Method 1 – Excess Method 2 – Excess a percentage of Incidence Screen Detection Invasive All Invasive All only cancers only cancers Cancers at ages 2% 5% 0% 6% 50-84 Cancers at ages 3% 8% 0% 8% 50-69 Screen-detected 7% 15% 0% 17% cancers
Both methods suggest that most of the overdiagnosis in the Norwegian programme was due to ductal carcinoma in situ.
Discussion
A substantial excess of cancers was observed in the age group 50-69 and smaller deficits at ages 70-84 for all cancers, invasive and ductal carcinoma in situ (figure 1). Most of the deficit was in cohorts who were aged less than 70 when screening was initiated, suggesting that this deficit is due to cancers detected earlier by screening.
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60-64 Observed
Results
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55-59
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.
Corresponding author: Stephen W. Duffy Insert partner Centre for Cancer Prevention, Queen Mary University of London Logo here Charterhouse Square, London, EC1M 6BQ
[email protected] www.yourwebaddresshere.ac.uk
