Not all cancers are equal.

Some grow fast and spread quickly, while others grow so slowly (or even not at all) they won’t cause any problems during a person’s lifetime. Even if they didn’t receive any treatment, someone with one of these slow-growing cancers wouldn’t be harmed by it or experience any symptoms.

When these harmless cancers are found, they’re said to be ‘overdiagnosed’.

The risk of overdiagnosis is an important consideration for cancer screening programmes, which test for very early signs of cancer, or changes that could lead to cancer, in people without symptoms. Although screening can save lives by finding cancers before they grow and spread, screening tests can’t currently tell us which cancers need treatment and which can be safely left alone.

That means screening can sometimes lead to unnecessary stress and worry by diagnosing cancers that won’t cause harm, as well as ‘overtreatment’, where people are exposed to the side effects of cancer drugs, surgery or radiotherapy they don’t actually need.

Overdiagnosis and overtreatment happen more often with certain types of cancer, so the UK National Screening Committee (UK NSC) – an independent body of experts who make recommendations to the four UK nations on cancer screening – pays close attention to the risks of overdiagnosis (and other possible harms) when evaluating screening tests and offering guidance on potential screening programmes.

The UK NSC has reviewed the best scientific evidence and concluded that overall the benefits of breast, bowel, cervical and targeted lung cancer screening outweigh the harms. However, over the last few decades overdiagnosis has been receiving more attention. And with new cancer detection technology on the horizon, ranging from blood tests to lollipops, it’s important that we learn more about when it occurs and how to minimise it.

What’s the evidence for overdiagnosis?

Some of the earliest evidence around overdiagnosis came from autopsy studies. Experts found undiagnosed cancers in people who had died from a cause other than cancer. This showed that in some cases people can live out their life without ever knowing a cancer is there, experiencing symptoms, or being harmed by it.

Other evidence for the phenomenon of overdiagnosis has come from digging into cancer statistics, particularly by comparing the number of people diagnosed with a cancer (incidence) with those that die from it (mortality).

Thyroid cancer in South Korea – a true epidemic?

Thyroid cancer is unlike many other cancers because it is often a small tumour that grows very slowly and is very treatable. However, this makes it more susceptible to overdiagnosis, especially through screening.

The UK NSC has never recommended a thyroid cancer screening programme and there has never been one in the UK. South Korea offers a useful case study of why not.

From 1999, hospitals in South Korea started to offer thyroid cancer screening as a cheap add-on to the country’s main screening programmes. Within a decade, the incidence of thyroid cancer in South Korea had increased dramatically.

If the number of cases of thyroid cancer were increasing, and if we assume they were all cancers that needed to be found and treated, we’d also expect a proportionate increase in the number of people dying from thyroid cancer.

That didn’t happen.

There are two possible explanations for this:

1. Thyroid cancer treatment improved dramatically at the same time as more people were being diagnosed, so lots more people were surviving the disease.
2. The cancers that were being picked up through screening were overdiagnosed.

The second explanation is far more likely as there were no major improvements to treatment during this time. In other words, simply scanning more thyroids picked up more of the slow-growing, harmless cancers that didn’t need to be found and treated in the first place.

Importantly, overdiagnosis can mean cancer survival statistics don’t always accurately represent the progress we’re making. For example, if more harmless cancers are found and treated, many more people will ‘survive’ the disease they’ve been diagnosed with, even though it might not have caused problems in the first place. So, it can be tricky to untangle the true impact of things like better treatments.

Thyroid cancer is much more prone to overdiagnosis than other types of cancer, but it’s not entirely unique. Other cancers prone to overdiagnosis include breast and prostate cancers.

Breast screening: saving lives and balancing harms

Breast screening picks up cancers at an early stage, when they have the best chance of being successfully treated. But it also comes with the risk of overdiagnosis, because some of those early invasive breast cancers wouldn’t have gone on to cause trouble in a person’s lifetime. Women need this information so they can make a fully informed decision about whether to go for screening.

Research has shown that for each woman whose life is saved through breast cancer screening, around three will be diagnosed with a breast cancer that would have never caused them any harm.

Because breast screening and diagnostic tests can’t yet tell the dangerous cancers that need treating from the harmless ones that don’t, doctors recommend treatment to everyone who is diagnosed with cancer after screening.

This means some people may experience serious and distressing consequences, including side effects from treatments that they did not need.

“Those three overdiagnosed women will be treated and will probably have a , radiotherapy and possibly hormonal therapy,” explains Professor Peter Sasieni, Cancer Research UK’s expert in cancer screening and epidemiology from Queen Mary University of London.

In this case, though, there are clear benefits tipping the balance in favour of screening. “If you think that for every three extra women treated for a screen-detected breast cancer that didn’t need to be found, another one will have her life saved by that treatment, it sounds very different,” Sasieni says.

“I think the language we use to talk about cancer is a big problem in society,” he adds. “I think most people still think of a cancer diagnosis as a death sentence.

“The grand challenge is to find something that tells us which cancers are harmless and which aren’t. If we could distinguish between them, we could ignore the harmless ones.”

Screening can save lives by diagnosing cancer at an earlier stage and, in some cases, preventing it from developing in the first place. But the decision to introduce new screening programmes has to be carefully thought through. That’s why we have the UK NSC.

Finding the balance

No screening test is perfect, but researchers all over the world, including many funded by Cancer Research UK, are working hard to improve our current tools and develop new ones.

We’re in exciting times when it comes to the potential for technology and innovation to diagnose cancer earlier, when it’s easier to treat. Multi-cancer tests have the potential to detect multiple cancer types from a single sample, but there’s a lot to learn before we can use them, including if they lead to overdiagnosis.

Targeted lung cancer screening is an example of a newer screening programme that uses an existing technology, CT scans, to help diagnose cancer earlier.

Most lung cancers are diagnosed at a late stage (stages 3 and 4), and this is largely to blame for lung cancer survival being so poor. Diagnosing more cancers early (at stage 1 and 2) is crucial for saving lives – and introducing targeted lung cancer screening is a way of finding cancer earlier.

However, there are still risks with screening. Lung cancer screening carries the risk of exposure to radiation from the CT scan (although low dose CT scans are used), as well as the risk of overdiagnosis. That’s why lung cancer screening is being targeted to people between the ages of 55-74 who smoke or used to smoke.

The UK NSC have recommended that for this group of people, lung cancer screening will offer more benefits than harms – and studies have shown that screening in this group would reduce lung cancer deaths.  Still, as ever, it’s important people take time to weigh up the pros and cons of lung screening when deciding if they want to take part

What next?

Screening can save lives by helping spot cancer early. Bowel and cervical screening can even prevent cancer from ever developing at all.

The UK’s national cancer screening programmes exist because research has shown that the benefits outweigh the harms for the population at large.

On the individual level, we all have our own choice to make about whether to take part in screening. Everyone will feel differently about the possible risks and benefits of screening and their own health. When you’re invited, be sure to read all the information in your invite to help you make the decision that feels right to you.

“Currently, in order to have the benefits of screening, there has to be some overdiagnosis,” says Sasieni.

If you’re unsure about what to do if you get an invitation for screening, visit our cancer screening information pages or speak to your doctor. We also have an article about making the screening process work for you.

Over the coming years, research into better tests and improved technology will help pick up cancers earlier.

From there, the challenge is working out how to tell the dangerous cancers from the harmless ones. But with overdiagnosis moving further into the spotlight, and with our strong commitment to improving cancer diagnosis, researchers are better placed than ever to tackle this.

And the more progress we make in understanding and diagnosing cancer, the more we’ll be able to change how we think about it – from a disease that always needs urgent treatment, to one that we can sometimes live with, or be unaware of and unaffected by.

This article was originally published in March 2018. It was updated with new information in November 2025.