A novel case–control design to estimate the extent of over‐diagnosis of breast cancer due to organised population‐based mammography screening

Abstract

Debate about the extent of breast cancer over-diagnosis due to mammography screening has continued for over a decade, without consensus. Estimates range from 0 to 54%, but many studies have been criticized for having flawed methodology. In this study we used a novel study design to estimate over-diagnosis due to organised mammography screening in South Australia (SA). To estimate breast cancer incidence at and following screening we used a population-based, age-matched case-control design involving 4,931 breast cancer cases and 22,914 controls to obtain OR for yearly time intervals since women's last screening mammogram. The level of over-diagnosis was estimated by comparing the cumulative breast cancer incidence with and without screening. The former was derived by applying ORs for each time window to incidence rates in the absence of screening, and the latter, by projecting pre-screening incidence rates. Sensitivity analyses were undertaken to assess potential biases. Over-diagnosis was estimated to be 8% (95%CI 2-14%) and 14% (95%CI 8-19%) among SA women aged 45 to 85 years from 2006-2010, for invasive breast cancer and all breast cancer respectively. These estimates were robust when applying various sensitivity analyses, except for adjustment for potential confounding assuming higher risk among screened than non-screened women, which reduced levels of over-diagnosis to 1% (95%CI 5-7%) and 8% (95%CI 2-14%) respectively when incidence rates for screening participants were adjusted by 10%. Our results indicate that the level of over-diagnosis due to mammography screening is modest and considerably lower than many previous estimates, including others for Australia.

Estimates of over-diagnosis of breast cancer due to population-based mammography screening in South Australia after adjustment for lead time effects

“…Our estimate of 8% for IBC is consistent with findings from randomized trials and the recent UK Independent Review, 20 and in agreement with results from our previous case-control study. 17 Our estimate may overstate the extent of over-diagnosis if projections have not accounted adequately for effects of increases in breast cancer risk factors in the decades following commencement of the screening programme.…”

Impact of extending screening mammography to older women Information to support informed choices

“…In Australia (where no screening mammography trials have been conducted), observational estimates of breast cancer mortality reduction for women screened from age 50–69 range from 21 to 49% . For overdiagnosis, figures for women aged 50–74 range from 15 to 42% . These local estimates, however, are subject to potential biases inherent in observational studies of screening.…”

Beyond the Mammography Debate: A Moderate Perspective

“…This is true however-a large proviso-only if we accept that, for reliable estimation, the length of follow-up required to assure no significant overestimation of overdiagnosis and to adjust for the potential bias from residual detection leadtime effects is 25 years or more of follow-up, as elegantly demonstrated by Stephen Duffy and Dharmishta Parmar 55 using a well-motivated exponential sojourn time model. Such a model helps to account for the wide variation in estimates of overdiagnosis [56][57][58][59][60][61] , but such follow-up is no easy requirement to meet.…”