Low-dose CT screening for lung cancer has cut deaths from that cancer by 20% to 24% in large trials. But screening only helps a clearly defined population: more tests does not automatically mean more lives saved.
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A striking figure that does not directly measure the effect of screening
| Route to diagnosis | Net 1-year survival |
|---|---|
| Detected through screening | 94% |
| After an urgent referral | 85% |
| After a referral by the family doctor | 82% |
| Found in the emergency department | 54% |
Colorectal cancer, cases diagnosed 2016 to 2020 in England. In 2019, 19.4% of cancers there were diagnosed through emergency presentation, and 22.5% in 2020.
Survival by stage: accurate, but not enough to measure the benefit of screening
| Cancer | Localised | Metastatic |
|---|---|---|
| Colorectal | 91.1% | 15.7% |
| Lung | 63.7% | 8.9%, and 53% of cases are found already metastatic |
| Melanoma | 100% | 34.6% |
| Prostate | 100% | 37.9% |
| Breast | over 99% | 32% |
Five-year relative survival by spread at diagnosis. These figures are accurate. The inference drawn from them is not always.
The gap between early and advanced stage embeds two well-known biases. Lead time first, since survival is counted from an earlier diagnosis, which mechanically lengthens measured survival without changing the date of death. Length time second, since slow-growing cancers are over-represented among those screening finds, an aggressive cancer being statistically less likely to be present on the day of the test.
The 75-point gap between localised and metastatic stage cannot therefore be turned into a benefit you should expect from screening. Randomised trials give a far more reliable estimate of what a screening programme actually achieves.
What randomised trials demonstrate: a real benefit, but a more modest one
| Trial | What is demonstrated | What is not |
|---|---|---|
| NELSON, CT lung screening, 2020 | Lung cancer mortality reduced by 24% in men. Striking stage shift, 46.8% of cancers at the earliest stage against 6.9% without screening, and 9.4% at metastatic stage against 45.7% | All-cause mortality is not the endpoint reached |
| NLST, lung screening, 2011 | Lung cancer mortality reduced by 20%, and all-cause mortality reduced by 6.7% | 18.5% of the cancers detected were overdiagnosis, tumours that would never have caused symptoms |
| NordICC, invitation to colonoscopy, 2022 | Colorectal cancer incidence reduced by 18% at 10 years | The mortality reduction does not reach significance. And participation was only 42% |
| MASAI, AI-read mammography, 105,934 women | Reading workload reduced by 44%, false positive rate unchanged | On the primary endpoint, interval cancers, the difference is not significant, ratio 0.88, p equals 0.41 |
That is the real order of magnitude. Minus 24% and minus 20% in lung cancer mortality, minus 18% in colorectal cancer incidence. These are solid and useful results. They are not the 40-point gap between screening and emergency presentation, and confusing the two is the most common error in the field.
Who performs the examination changes what it finds
This is the most decisive and the least known element. Across 314,872 colonoscopies performed by 136 gastroenterologists, the detection rate for precancerous lesions ranges from 7.4% to 52.5% depending on the operator, a 7-fold spread for the same examination, within the same care organisation, with the same equipment.
And that spread shows up in mortality. Among patients of top-quintile operators, observed interval-cancer mortality is more than half lower than in the bottom quintile, in an observational comparison between operators. Every additional point of detection rate goes with a 3% lower cancer risk.
The starting point is in any case known across the profession. A meta-analysis of 43 publications and more than 15,000 back-to-back colonoscopies in the same patients establishes an adenoma miss rate of 26%, one in four, and 27% for sessile serrated polyps. What distinguishes one centre from another is not pretending that figure does not exist, it is what is put in place against it.
Why patients diagnosed quickly can sometimes have a poorer outlook
There is a well-documented and rarely mentioned phenomenon. In several cancers, the patients diagnosed fastest are also those whose disease is the loudest, therefore the most aggressive. A systematic review of 177 articles shows that only 7 studies account for it. A correlation between delay and survival is therefore never proof that the delay alone is the cause, and we will not present it as such.
What the studies do not allow us to claim
Nothing entitles us to write that screening halves the risk of dying, because randomised trials give reductions of 18 to 24%. We will not promise that a normal test guarantees the absence of disease, because one adenoma in four is missed at standard colonoscopy. And we will not stay silent on overdiagnosis, which affects nearly one screened lung cancer in five.
A cancer found through screening and a cancer found in the emergency department are therefore two very different situations. In England, 22.5% of cancers were still diagnosed through emergency presentation in 2020.
In practice, if this concerns you
You set out your problem, your symptoms or your question. Reports, test results and images already available can be reviewed before you arrive, so that the pathway is prepared. The aim is not to run every possible test, but to select those that can genuinely help confirm a hypothesis, rule out a risk or guide a decision. The results are brought together in one summary and the next steps are organised. Follow-up is then coordinated.
Arrange my check-up or second opinion
Need a medical opinion quickly? For a concern that does not call for the emergency services, you can ask to speak to a doctor at any time through Noria Health Hub. This on-demand medical consultation is billed separately from the work-up pathway.
Further reading
The companion articles.
Who” rel=”noopener” target=”_blank”>https://noriahealth.com/blog/who-performs-your-examination-changes-what-it-finds/”>Who performs your examination changes what it finds
What” rel=”noopener” target=”_blank”>https://noriahealth.com/blog/what-a-health-assessment-can-actually-find/”>What a health assessment can actually find
Sources
NHS England, National Disease Registration Service, Routes to Diagnosis, cancers diagnosed 2016 to 2020. https://digital.nhs.uk/ndrs/our-work/ncras-work-programme/routes-to-diagnosis
Surveillance, Epidemiology and End Results Program, National Cancer Institute, Cancer Stat Facts, 2014 to 2020. https://seer.cancer.gov/statfacts/
de Koning HJ et al., NELSON, Reduced lung-cancer mortality with volume CT screening, New England Journal of Medicine, 2020. https://doi.org/10.1056/NEJMoa1911793
Aberle DR et al., NLST, Reduced lung-cancer mortality with low-dose computed tomographic screening, New England Journal of Medicine, 2011. https://doi.org/10.1056/NEJMoa1102873
Patz EF et al., Overdiagnosis in low-dose computed tomography screening for lung cancer, JAMA Internal Medicine, 2014. https://doi.org/10.1001/jamainternmed.2013.12738
Bretthauer M et al., NordICC, Effect of colonoscopy screening on risks of colorectal cancer and related death, New England Journal of Medicine, 2022. https://doi.org/10.1056/NEJMoa2208375
Corley DA et al., Adenoma detection rate and risk of colorectal cancer and death, New England Journal of Medicine, 2014. https://doi.org/10.1056/NEJMoa1309086
Zhao S et al., Magnitude, risk factors, and factors associated with adenoma miss rate of tandem colonoscopy, Gastroenterology, 2019. https://pubmed.ncbi.nlm.nih.gov/30738046/
Lång K et al., MASAI, Artificial intelligence-supported screen reading versus standard double reading, The Lancet Digital Health then The Lancet Oncology, 2023 and 2025. https://doi.org/10.1016/S1470-2045(23)00298-X
Neal RD et al., Is increased time to diagnosis and treatment in symptomatic cancer associated with poorer outcomes, British Journal of Cancer, 2015. https://www.nature.com/articles/bjc201548
Hanna TP et al., Mortality due to cancer treatment delay, systematic review and meta-analysis, British Medical Journal, 2020. https://www.bmj.com/content/371/bmj.m4087