Around a third of women diagnosed with invasive breast cancer or ductal carcinoma in situ (DCIS – abnormal breast tissue) through screening in Denmark are likely to receive unnecessary treatment, according to new estimates.
When looking only at invasive breast cancer cases, the chance of overdiagnosis or unnecessary treatment was found to be lower.
The findings, published in Annals of Internal Medicine, highlight the problem of overdiagnosis, where slow-growing tumours that wouldn’t cause a woman any harm are picked up.
These women are usually given unnecessary treatment because there isn’t a reliable way to tell which cancers have been overdiagnosed.
Fiona Osgun, Cancer Research UK’s senior health information officer, said the study reinforces that breast screening has harms as well as benefits.
“Women need to be aware of these when deciding whether or not to take up their invitations to screening,” she added.
The study looked at Denmark’s screening programme, which offers mammograms to women aged between 50 and 69.
Researchers compared the number of tumours found in women of different age groups across different regions of Denmark before and after screening was introduced.
They looked at information from more than 95,000 Danish women diagnosed with invasive breast cancer or DCIS between 1980-2010.
But the study didn’t look at the effect of screening on lives saved, where other research has shown that screening brings benefits.
“Overall the evidence shows that breast screening reduces deaths from the disease but also causes some women to be overdiagnosed,” said Osgun. “This means screening finds a slow-growing cancer that would never have caused a problem during a woman’s lifetime but leads to unnecessary treatment.”
“Estimates vary as to how many breast cancers are overdiagnosed through screening, but most experts agree that overdiagnosis is a problem.
“The UK Independent Breast Screening Review in 2012 estimated that around 1,300 lives are saved each year in the UK but around 4,000 women are overdiagnosed.
The study’s authors said screening was important, but warned of an over-reliance and suggested the focus should be on preventing breast cancers from developing.
Professor Paul Pharoah, from the University of Cambridge, said the findings confirmed what was already known, and stressed that screening remained a personal choice.
He said: “Whether or not the benefit is worth the harm is an individual decision and different women will make different choices.”
Osgun agreed: “Breast screening is a personal choice, so it’s essential that women who are invited have good quality, balanced information about the harms and benefits to help them decide whether they want to take up the offer.”
In the UK the NHS Breast Screening Programme invites all women aged between 50 and 70 for screening every 3 years.
Jørgensen, K. J., et al. (2017). Breast Cancer Screening in Denmark. A Cohort Study of Tumor Size and Overdiagnosis. Ann Intern Med. DOI: 10.7326/M16-0270