In the United Kingdom, breast cancer accounts for 31% of all new cancers diagnosed in each year. In the last decade, cases having increased by 4%. But, despite the rise in new cases, deaths caused by breast cancer are declining. This is in part due to early diagnosis of breast cancer through the National Health Service (NHS-) Breast Screening Program.

The NHS Breast Screening Program invites women between 50-70 years of age who are registered with a General Practitioner (GP) or Primary Care Physician (PCP) for a screening every three years.

Breast cancer screening uses x-rays called mammograms to check breasts for signs of cancer and despite the benefits of screening, there remains some debate over the potential harms of screening – notably over-diagnosis.

In this case, overdiagnosis is not the same as when a test finds something abnormal that turns out not to be cancer (a false-positive). Over-diagnosis refers to a diagnosis of a real cancer which grows so slowly that it would never have actually given any symptoms.

Over-diagnosis may be a consideration when women are thinking about whether or not to participate in breast cancer screening. Until now, estimates of overdiagnosis have varied widely, ranging from less than 5% of screen detected cancers to more than 30%.

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When a cancer is diagnosed, phycicians cannot tell whether it is over-diagnosed or not, so all cancers need treatment.

Reliable estimates
It is important to have reliable estimates of over-diagnosis in order to make an informed decision on whether to be screened or not. To quantify over-diagnosis in the NHS Breast Screening Program, researchers at Queen Mary undertook a study of 57,493 breast cancer cases diagnosed in 2010 or 2011, matched with 105,653 controls, with cases defined as women diagnosed at ages 47–89 with primary breast cancer, invasive or ductal carcinoma in situ (DCIS). The study was funded by the United Kingdom National Institute of Health and Care Research (NIHR-) Policy Research Program.[1]

They estimated the effect of screening on breast cancer risk, and the results were combined with national incidence data to estimate absolute rates of over-diagnosis. Over-diagnosis was calculated as the cumulative excess of cancers diagnosed in women aged 50-77 attending three-yearly screening between ages 50 and 70 compared with women attending no screens.

The estimated number of cases overdiagnosed in women attending all screens in the programme was 679.3 per 100,000 without adjustment for self-selection bias and 261.2 per 100,000 with adjustment. These corresponded to an estimated 9.5% of screen-detected cancers overdiagnosed without adjustment and just under 3 per 1,000, corresponding to an estimated 3.7% of screen detected cancers over-diagnosed with adjustment for self-selection.

Stephen W. Duffy, BSc, MSc, CStat, Professor of Cancer Screening, Queen Mary University of London

Lower than suggested
Based on the outcome, the authors of the study concluded that the NHS Breast Screening Program confers just a modest levels of over-diagnosis, which is considerably lower than has been suggested in the past.

“These results provide some reassurance that participation in the NHS Breast Screening Program confers only a low risk of an over-diagnosed breast cancer,” noted Stephen W. Duffy, BSc, MSc, CStat, Professor of Cancer Screening, Queen Mary University of London and joint lead investigator of the study.

“Along with the results of our previous study of the effect of screening on breast cancer mortality, this indicates that the benefit of screening in preventing deaths from breast cancer outweighs the small risk of over-diagnosis,” Duffy added.

Professor Peter Sasieni Ph.D., is the Academic Director of the Kings Clinical Trials Unit at King’s College London. He is an expert in cancer prevention, clinical trials and cancer screening, and his work influences both policy and practice.

A complex issue
“Overdiagnosis is quite a complex issue. Had we stopped following women at age 70, nearly 1% of screened women would seemingly have been over-diagnosed, but by wating another 7 years, more never-screened women will be diagnosed with breast cancer and the three-quarters of the excess cancers turn out to have been diagnosed early rather than over-diagnosed,” said Peter Sasieni, PH.D., Academic Director of the Clinical Trials Unit and Professor of Cancer Prevention at King’s College London, London, United Kingdom, and joint lead investigator.

“What this means is that most previous studies have overestimated the harms of participating in breast cancer screening up to the age of 70,” Sasieni concluded.

[1] Maroni R, Massat NJ, Parmar D, Dibden A, Cuzick J, Sasieni PD, Duffy SW. A case-control study to evaluate the impact of the breast screening programme on mortality in England. Br J Cancer. 2021 Feb;124(4):736-743. doi: 10.1038/s41416-020-01163-2. Epub 2020 Nov 23. PMID: 33223536; PMCID: PMC7884709.

Featured image: Breast cancer screening. Photo courtesy: © 2016 – 2022 National Cancer Institute on Unsplash

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