Overdiagnosis in screening mammography in Denmark: population based cohort study

Abstract

Objective To use data from two longstanding, population based screening programmes to study overdiagnosis in screening mammography.Design Population based cohort study.Setting Copenhagen municipality (from 1991) and Funen County (from 1993), Denmark.Participants 57 763 women targeted by organised screening, aged 56-69 when the screening programmes started, and followed up to 2009. Main outcome measuresOverdiagnosis of breast cancer in women targeted by screening, assessed by relative risks compared with historical control groups from screening regions, national control groups from non-screening regions, and historical national control groups. ResultsIn total, 3279 invasive breast carcinomas and ductal carcinomas in situ occurred. The start of screening led to prevalence peaks in breast cancer incidence: relative risk 2.06 (95% confidence interval 1.64 to 2.59) for Copenhagen and 1.84 (1.46 to 2.32) for Funen. During subsequent screening rounds, relative risks were slightly above unity: 1.04 (0.85 to 1.27) for Copenhagen and 1.14 (0.98 to 1.32) for Funen. A compensatory dip was seen after the end of invitation to screening: relative risk 0.80 (0.65 to 0.98) for Copenhagen and 0.67 (0.55 to 0.81) for Funen during the first four years. The relative risk of breast cancer accumulated over the entire follow-up period was 1.06 (0.90 to 1.25) for Copenhagen and 1.01 (0.93 to 1.10) for Funen. Relative risks for participants corrected for selection bias were estimated to be 1.08 for Copenhagen and 1.02 for Funen; for participants followed for at least eight years after the end of screening, they were 1.05 and 1.01. A pooled estimate gave 1.040 (0.99 to 1.09) for all targeted women and 1.023 (0.97 to 1.08) for targeted women followed for at least eight years after the end of screening. ConclusionsOn the basis of combined data from the two screening programmes, this study indicated that overdiagnosis most likely amounted to 2.3% (95% confidence interval −3% to 8%) in targeted women. Among participants, it was most likely 1-5%. At least eight years after the end of screening were needed to compensate for the excess incidence during screening.

Benefit-to-harm ratio of the Danish breast cancer screening programme

“…The methodology presented by the EUROSCREEN group was used to estimate the number of breast cancer deaths prevented and the number of overdiagnosed cases 1. As the EUROSCREEN group estimation is based on cumulative rates for age 50 to 79, the results of our present study will differ from our previously published estimates on number of breast cancer deaths prevented4 and on number of overdiagnosed cases5 using other methods. The results were presented for every 1,000 women aged 50 years invited to screening until age 69 years and followed until age 79 years.…”

“…The estimates of breast cancer mortality reduction and overdiagnosis were extracted from our previous studies, which used the difference‐in‐differences method based on four cohorts with individual follow‐up data 3, 4, 5. The methodology presented by the EUROSCREEN group was used to estimate the number of breast cancer deaths prevented and the number of overdiagnosed cases 1.…”

“…For overdiagnosis for invited women, we used the increase in the pooled cumulative incidence of breast cancer (invasive and DCIS) of 2.3% (95% CI: −3 to 8%) reported by Njor et al 5. For screened women, overdiagnosis amounted to 1% in Funen and to 5% in Copenhagen according to Njor et al 5.…”

“…For screened women, overdiagnosis amounted to 1% in Funen and to 5% in Copenhagen according to Njor et al 5. Person‐years of women not followed beyond screening age are not relevant for estimation of overdiagnosis.…”

“…We found implementation of screening to be followed by a 25% decrease in breast cancer mortality in Copenhagen3 and by a 22% decrease in Funen 4. We found overdiagnosis, including invasive and ductal carcinoma in situ (DCIS), to account for 2.3% of the breast cancer cases in Copenhagen and Funen 5. This article aims to estimate the benefit‐to‐harm ratio of breast cancer screening in Denmark using the long‐term cohort data on the screening outcomes.…”