This month, it was reported that England’s existing screening programme for bowel cancer for 60 to 74-year-olds had prevented 20,000 cases of bowel cancer over the past 10 years. This comes amid an ongoing effort in England to extend bowel cancer screening to those aged between 50 and 59. And in June, the NHS national director for cancer welcomed a new blood test currently being trialled in the UK that can detect 50 types of the disease.
More cancer screening seems like unqualified good news: generally, the earlier cancer is diagnosed, the higher the chance of successful treatment. However, there are harms that come with screening, too, both for people being screened and for the NHS as a whole that should be mitigated against. I am certainly not discouraging people from taking up screening offers, but expansion of screening programmes must come with concrete plans about delivery. This requires extra NHS fundingand capacity.
The issue is that the large majority of people who are screened don’t have cancer. But some will nonetheless receive an ambiguous or false positive result, sending them for further testing. This is where screening can actively harm. Almost all of them will have cancer ruled out on further testing, but this can take several weeks and studies have shown that receiving a “false positive” from a cancer screening test can significantly increase anxiety and stress, lasting a few weeks to months.
This is most common in breast and prostate cancers but can also happen for others. In fact, prostate cancer screening is so unreliable that it’s not even offered by the NHS currently. Researchers must keep refining screening tests to make sure fewer people incorrectly test positive, and the NHS must make careful decisions about who should be tested and when.
Screening is also not a one-off, but is performed regularly. The chances of being wrongly sent for testing, or worse, wrongly treated, can become relatively high over the years. One recent study estimated that a 50-year-old starting bowel cancer screening today had about a 15% chance of being wrongly flagged for a colonoscopy by the time they were 74. For breast cancer, it is estimated that about 3 in every 200 women going through screening will be over-diagnosed with the disease.
From an NHS point of view, screening is very resource-intensive to deliver. Each person falsely flagged by screening requires further, more intensive, hospital tests – and uses hospital resources that can’t be used for someone else who has been correctly flagged.
If the NHS is going to massively expand screening, it needs sufficient resources to deliver not just the screening itself, but also the subsequent gold-standard testing and treatments. But the outlook is not good: latest NHS statistics show that cancer diagnosis and treatment targets have been breached more than a million times since October 2022. Diagnostic equipment such as MRI or CT scanners, and the staff to run them, are expensive resources and already stretched, with significant wait times.
Much less harmful in terms of screening is bowel cancer: it consists of sending a stool sample, taken at home, off for testing. If there are traces of blood in the sample, then you are offered a colonoscopy. It is almost impossible to be wrongly diagnosed with bowel cancer during a colonoscopy. The potential harm to a person from a bowel cancer screening is mainly limited to an unnecessary colonoscopy, while there is a large benefit in removing pre-cancerous growths and identifying cancers early.
But the NHS implications are a different matter. Extending bowel cancer screening in England to people in their 50s means almost doubling the number of people screened each year, to about 16 million. Given that fiftysomethings are about half as likely to develop bowel cancer or adenomas as people in their 60s, it also means screening many more people who do not have cancer or pre-cancerous growths.
Even if the chance of incorrectly flagging a stool sample for a colonoscopy is only 1% (a conservative estimate), this could mean thousands more people referred each year for a colonoscopy they do not need. And colonoscopy services are already very stretched, coping with the pandemic backlog: around 40% of people in England are now waiting more than six weeks for a colonoscopy, up from around 35% in 2022. As it stands, it’s not clear how the NHS will be able to deliver these services to the new, lower age group in a timely manner without a large increase in funding and capacity.
There are similar issues with the new blood test for a range of 50 potential cancers. The test is intended to help diagnose people with non-specific symptoms that could signal cancer. But the NHS is trialling the test on 140,000 people without any symptoms and, if that trial is successful, hopes to extend that to a further million people over the next few years. Yet a recent UK study found that among participants with non-specific symptoms the false positive rate was about 1.5%. This means that thousands of people without cancer (1.5% of almost all those 1 million people the NHS is trialling) could be wrongly flagged for further gold-standard testing.
For the NHS, this means adding tens of thousands more people to the cancer diagnostic pathway. This will increase wait times for everyone – including those at a more advanced cancer stage – and reduce available resources elsewhere in the system.
The benefits, harms and costs of screening need to be transparently communicated to the public. At the moment, more screening is announced and reported as an obvious triumph, but if we want to actually improve cancer treatment in the UK, we have to have a deeper discussion on the best use of (currently) scarce resources.
Christina Pagel is professor of operational research, UCL
