Over- and underdiagnosis in BreastScreen NOrway

A PhD project will investigate aspects of over- and underdiagnosis in BreastScreen Norway. The goal of the project is to add to what is known about the potential harms of mammographic screening.

Published 10 October 2018, updated  2 January 2020.

Background 

Breast cancer is the most common type of cancer among women in Norway. about 3,500 new cases are diagnosed each year. Screening with mammography can detect breast cancer in an early stage. The goal of screening is to reduce breast cancer deaths and to offer women less aggressive treatment; however, screening also carries some risks. 

In Norway, women aged 50-69 years old are invited to BreastScreen Norway, the national public screening program for breast cancer. Women who are invited to screening need information on both the benefits and harms of screening to make an informed choice whether to participate.

Purpose

The principle behind mammography screening is to examine many otherwise healthy women in order to diagnose the few who have breast cancer. It is important to minimize the risks associated with screening, and to avoid treating conditions that do not require treatment.

BreastScreen Norway works continuously to minimize the risks of mammographic screening, including the risk for under- or overdiagnosis of breast cancer, among others.

Underdiagnosis can occur when a breast tumour is overlooked. These tumours can later be detected between two screening examinations because of symptoms. Tumours detected between two screening examinations often have a worse prognosis than breast cancers that are detected as a result of screening. Underdiagnosis is not as well studied as many other aspects of mammographic screening because only a small percentage of breast cancer cases in a screening program are underdiagnosed. This means that many mammography images are needed to perform a robust study. Obtaining these data requires a lot of resources.

On the other hand, overdiagnosed tumours are small, slow growing tumours. These tumours are cancerous but will never cause symptoms and, as a result, would never be detected without screening. It is currently not possible to determine which specific tumours are overdiagnosed. Because of this, overdiagnosis can lead to overtreatment. How often overdiagnosis occurs is highly discussed because there is no agreement on how to best estimate it. Some studies estimate that mammographic screening leads to a very low rate of overdiagnosis. Others have estimated relatively high rates of overdiagnosis.

Awareness and knowledge about breast cancer, particularly under- and overdiagnosis, is important for women when they make a decision whether to participate in BreastScreen Norway.

Through this project, we will describe tumour characteristics and survival among women with potentially under- or overdiagnosed cancers in BreastScreen Norway. Additionally, we will estimate the rate of overdiagnosis in the program, using individual-level data that includes information on socioeconomic factors, to obtain a more precise estimate of the rate of overdiagnosis in Norway.

We will also study the level of awareness and knowledge that women and primary care doctors have about breast cancer, including under-and overdiagnosis. The aim of this is to determine whether women invited to participate in screening have adequate knowledge, and have access to health care providers with adequate knowledge, to make an informed choice whether to participate in BreastScreen Norway.

Planned studies

Study 1: Describe tumour characteristics and survival for potentially under- or overdiagnosed interval and screen-detected breast cancers. Retrospective cohort study, based on individual-level data.

Study 2: Estimate the rate of overdiagnosis in the screening program using individual-level data, after adjustment for socioeconomic factors. This study will give new knowledge about how socioeconomic factors can affect rates of overdiagnosis. Retrospective cohort study, based on individual-level data.

Study 3: To describe Norwegian women’s and primary care doctors’ awareness and knowledge about under- and overdiagnosis and breast screening in general. Cross-sectional study based on an online questionnaire sent to women 50-69 years old and primary care doctors of all ages in Norway.

Research team

PhD candidate Kaitlyn Tsuruda, MSc, Cancer Registry of Norway, will carry out this project.

The primary investigator and supervisor of this project is Solveig Hofvind, head of the mammography section at the Cancer Registry of Norway, and professor of radiography at Oslo Metropolitan University.

Marit Veierød, professor in biostatistics at the University of Oslo, is the co-supervisor.

External collaborators include:

  • Stephen Duffy, Queen Mary University of London
  • Ragnhild Sørum Falk, Oslo University Hospital
  • Signe Flottorp, Norwegian Institute of Public Health
  • Marit Solbjør, Norwegian University of Science and Technology
  • Livia Giordano, Centre for Cancer Prevention
  • Nehmat Houssami, University of Sydney
  • Håkan Jonsson, Umeå University
  • Wendy Wu, Umeå University
  • Jennifer Payne, Dalhousie University

Publications

Terminal digit preference: a source of measurement error in breast cancer diameter reporting. Tsuruda KM, Hofvind S, Akslen LA, Hoff SR, Veierød MB.
Acta Oncol. 2019 Sep 30:1-8. doi: 10.1080/0284186X.2019.1669817. [Epub ahead of print]