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Trial Lecture – time and place
See Trial Lecture.
Adjudication committee
- First opponent: Professor Stephen W. Duffy, Queen Mary University of London, UK
- Second opponent: Professor Emeritus Lars Holmberg, King’s College London, UK and Uppsala University, Sweden
- Third member and chair of the evaluation committee: Professor Vessela Nedelcheva Kristensen, University of Oslo
Chair of the Defence
Professsor Espen Bjertness, University of Oslo
Principal Supervisor
Senior Researcher Harald Weedon-Fekjær, Oslo University Hospital
Summary
Population-based mammography screening programs have been introduced with the aim of reducing breast cancer mortality. While randomized screening trials have shown a clear effect of screening on breast cancer mortality, population trend studies have often found no effect. With early detection of tumors by screening, the question has also been raised as to whether all the preclinical breast tumors have a clinical potential.
Using data from the Cancer Registry of Norway, the aims of the thesis were to: i) Study, by simulations, how the effect of screening is expected to appear in population wide breast cancer mortality, ii) Estimate the frequency of non-progressive screen-detected breast cancers and iii) Estimate the total amount of over-diagnosis in BreastScreen Norway, by statistical modelling and analysis of Norwegian breast cancer incidence around screening introduction.
We found that the effect of screening is expected to appear very gradually at population level, mainly since a large proportion of breast cancer deaths soon after the introduction of screening are among women already diagnosed with breast cancer before screening initiation. The statistical power to detect an effect of screening from population time trends is, in practice, very low. If screening reduces the breast cancer mortality by 33% among non-symptomatic women attending screening, an 18% reduction in breast cancer mortality is first expected for women of screening age (50-69 years of age) 20 years after initiation of screening.
Using statistical modelling of breast cancer incidence around screening introduction, we found that 16% of screen-detected tumors are not expected to manifest clinically by the time the women reach 85 years of age.
In addition to the presence of non-progressive tumors, some women die of other causes before progressive tumors become clinically apparent. Thus, we estimate that 18% of screen-detected breast cancer tumors, including DCIS, are in practice overdiagnosed.
Additional information
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