New analysis finds breast screening overdiagnosis may be lower than feared — but access to timely diagnosis remains unequal.

Breast cancer is the world’s most common cancer among women. In 2022, an estimated 2.3 million women were diagnosed and about 670,000 died, according to the World Health Organization (WHO).

Early detection and effective treatment can prevent many premature deaths. Yet screening also carries a real risk, that is finding cancers that would never have caused symptoms or shortened a woman’s life. This is called overdiagnosis, and it can lead to unnecessary surgery, radiotherapy, hormone treatment and the anxiety of a cancer diagnosis.

A new analysis of eight major mammography trials suggests that this harm may be less common than some earlier estimates indicated. The study, published in the Journal of the National Cancer Institute, estimates that overdiagnosis in mammography screening may be below 5% when trial data are interpreted over a sufficiently long period.

The finding does not settle every question about screening. But it shifts attention towards a more immediate public-health failure that many women still cannot obtain high-quality screening, prompt diagnostic tests or cancer treatment when they need them.

What is happening

Mammography uses low-dose X-rays to identify breast abnormalities before they can be felt. The purpose is to find cancers at an earlier, more treatable stage.

Overdiagnosis occurs when screening detects a cancer that would not have caused symptoms or become life-threatening during the person’s natural lifespan. The cancer is real under the microscope, but treating it may not improve that woman’s health.

This distinction matters because screening initially increases the number of cancers recorded. It finds some cancers earlier than they would otherwise have been diagnosed. A rise in diagnoses, therefore, is not automatically evidence that screening is finding large numbers of harmless cancers.

The researchers behind the new meta-analysis reviewed all eight randomised mammography-screening trials across multiple countries, including studies from Sweden, the United Kingdom, Canada and the United States. They compared the pattern of diagnoses in those trials with long-term Danish screening data.

Their conclusion is that some past estimates may have counted cancers diagnosed earlier through screening as overdiagnosis before the control groups had enough time to “catch up”.

Why it matters

Debate over overdiagnosis has shaped breast-screening policy for decades. Some estimates from earlier trial analyses suggested that 30% to 50% of screen-detected breast cancers could represent overdiagnosis.

If true, such figures would substantially alter the balance between screening’s benefits and harms. Women deserve clear information about that balance before they decide whether to attend.

But estimates are highly sensitive to study design. They can be distorted if women in the comparison group later receive mammograms, if the screened group has more rounds of screening, or if follow-up ends too soon.

The new research states that, once these factors are considered, the data are consistent with an overdiagnosis rate below 5%.

That is an important finding, but it should not be read as a reason for complacency. Breast cancer outcomes remain deeply unequal. Women who have access to organised screening, rapid biopsy services, specialist surgery and modern medicines are more likely to be diagnosed early and survive.

Those living far from specialist centres, facing financial barriers or navigating overstretched public services may reach care later, when treatment is harder and less effective.

What drives the problem

Breast cancer is not one disease with one cause. Age, family history, inherited genetic variants, alcohol use, obesity after menopause, reproductive history and hormone exposure can all affect risk.

Many women diagnosed with breast cancer, however, have no obvious risk factor beyond being female and getting older.

The much larger divide is created by health systems.

A screening invitation has limited value if women cannot travel to an imaging centre, afford follow-up tests, take time off work or obtain a timely appointment after an abnormal result. Screening also depends on accurate interpretation, quality assurance, pathology services and a reliable route into treatment.

The risk of overdiagnosis must therefore be considered alongside the risk of underdiagnosis and delayed diagnosis. In settings without organised screening programmes, the central problem is often not too much detection, but cancer being found only after a lump, pain or other symptoms bring a woman to care.

What works

Evidence from randomised trials shows that mammography screening can reduce deaths from breast cancer in the age groups invited to participate. The size of the benefit varies according to age, screening interval, programme quality and access to treatment.

The strongest programmes do more than offer mammograms. They ensure women receive understandable information, recall those with abnormal results quickly, investigate findings without unnecessary delay, and connect confirmed cases to multidisciplinary cancer care.

The new study also points to a practical lesson for researchers and policy-makers: screening should be judged over time, not by counting the immediate rise in diagnoses after a programme begins.

“When screening is introduced, the number of breast cancer diagnoses initially rises because cancers are detected earlier than they would have been without screening,” said Professor Elsebeth Lynge of the University of Copenhagen, one of the study’s authors. “Over time, this should be followed by a drop, as some of these cancers would otherwise have been diagnosed later.”

That pattern is central to separating earlier detection from overdiagnosis.

Who is left out — and why

Even within countries with established screening programmes, participation is often lower among women on low incomes, women in rural areas, migrants, ethnic minorities, women with disabilities and those with insecure immigration status.

The barriers are often ordinary but consequential like transport costs, unpaid leave, childcare, language, fear of a diagnosis, limited health literacy and previous experiences of discrimination in healthcare.

Women with symptoms should not wait for a routine screening appointment. A new breast lump, persistent change in breast shape, skin dimpling, nipple changes or unexplained discharge needs clinical assessment. Screening is designed for people without symptoms; it is not a substitute for diagnostic care.

For women already diagnosed, inequity can continue through treatment. Delays in surgery, radiotherapy, pathology results and access to medicines can turn a potentially treatable illness into a more dangerous one.

A question of informed choice

WHO has stressed that early diagnosis and prompt treatment are essential parts of breast-cancer control, alongside efforts to reduce avoidable risk and strengthen health systems.

The new analysis does not erase the harms of screening. Some women will still face false-positive results, extra tests, biopsies and treatment for cancers that may never have harmed them. Those harms should be explained honestly.

But Professor Sisse Helle Njor of the University of Southern Denmark and Lillebælt Hospital, who led the study, said the evidence should not be interpreted as showing that overdiagnosis is inevitably widespread.

“Randomized trials have often been cited as evidence that overdiagnosis is a substantial problem,” she said. “Our study shows that this interpretation is not as straightforward as it may seem.”

The obligation is not to persuade women into screening or frighten them away from it. It is to give them evidence they can understand, including the likely benefits, possible harms and the limits of any test.

Policy and accountability

Health authorities should be cautious about using a single overdiagnosis estimate to justify either expanding or restricting screening. The benefit-harm balance differs between populations and depends heavily on programme quality.

Governments that offer breast screening must fund the full pathway, for example, invitations that reach marginalised communities, trained radiographers and radiologists, reliable imaging equipment, rapid diagnosis, pathology, surgery, radiotherapy, medicines and follow-up care.

They must also publish transparent data. Participation rates, waiting times after abnormal mammograms, stage at diagnosis, treatment access and outcomes should be reported by geography, income, ethnicity and other measures of disadvantage where lawful and appropriate. A national average can conceal serious gaps.

Research, too, requires accountability. Estimates of overdiagnosis should make clear how long women were followed, whether comparison groups later received screening, and whether analyses distinguish early diagnosis from detection of cancers that would never have caused harm.

What must change

The new study offers reassurance that mammography overdiagnosis may be substantially lower than the highest estimates that have shaped public debate. It does not make screening risk-free, and it does not remove the need for frank consent.

But the greater injustice remains clear that too many women are still denied the chance to benefit from early diagnosis and timely treatment because care depends on where they live, what they earn or whether they can navigate a fragmented system.

Breast-cancer screening should never be reduced to a test alone. Its value rests on what follows: accurate information, rapid diagnosis, appropriate treatment and equal access to every part of that pathway.