Abstract
BackgroundMammography screening is used to detect breast cancer at an early treatable stage, reducing breast cancer mortality. Traditionally, breast cancer has been seen as a disease with only progressive lesions, and here we examine the validity of this assumption by testing if incidence levels after introducing mammography screening can be reproduced assuming only progressive tumors.MethodsBreast cancer incidence data 1990–2009 obtained from the initially screened Norwegian counties (Akershus, Oslo, Rogaland and Hordaland) was included, covering the time-period before, during and after the introduction of mammography screening. From 1996 women aged 50–69 were invited for biennial public screening. Using estimates of tumor growth and screening sensitivity based on pre-screening and prevalence screening data (1990–1998), we simulated incidence levels during the following period (1999–2009).ResultsThe simulated incidence levels during the period with repeated screenings were markedly below the observed levels. The results were robust to changes in model parameters. Adjusting for hormone replacement therapy use, we obtained levels closer to the observed levels. However, there was still a marked gap, and only by assuming some tumors that undergo regressive changes or enter a markedly less detectable state, was our model able to reproduce the observed incidence levels.ConclusionsModels with strictly progressive tumors are only able to partly explain the changes in incidence levels observed after screening introduction in the initially screened Norwegian counties. More complex explanations than a time shift in detection of future clinical cancers seem to be needed to reproduce the incidence trends, questioning the basis for many over-diagnosis calculations. As data are not randomized, similar studies in other populations are wanted to exclude effect of unknown confounders.
Highlights
Mammography screening is used to detect breast cancer at an early treatable stage, reducing breast cancer mortality
In Norway the introduction of organized biennial mammography screening in the age group 50–69 years was associated with a reduced breast cancer mortality [1], and a marked increase in the incidence of invasive breast cancer in women invited to participate in the screening program [2, 3]
The breast cancer incidence may change due to causes not related to Westvik et al BMC Cancer (2018) 18:209 screening, such as exposure to postmenopausal hormone replacement therapy (HRT)
Summary
Mammography screening is used to detect breast cancer at an early treatable stage, reducing breast cancer mortality. The aim of breast cancer screening is to detect cancers at an early treatable state to reduce mortality. In Norway the introduction of organized biennial mammography screening in the age group 50–69 years was associated with a reduced breast cancer mortality [1], and a marked increase in the incidence of invasive breast cancer in women invited to participate in the screening program [2, 3]. High detection rates over time may reflect that mammography is associated with overdiagnosis, i.e. the diagnosis of tumors that will never cause clinical disease in the life time of the women
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