National Lung Screening Trial (NLST): What the Lung Cancer Trial Found

Screening, not treatment

This trial was not about a drug. It was about looking for lung cancer in people who felt perfectly well, to see whether finding it earlier meant fewer of them died of it.

That question had been asked before with chest X-rays, and the answer had been no. Low-dose CT scans the whole chest in slices rather than flattening it into a single image, and by 2002 they were picking up small tumours that X-rays missed. Whether that translated into fewer deaths was still unknown.

Who was screened, and how often

Field	Detail
Trial	National Lung Screening Trial (NLST)
Identifier	NCT00047385
Design	Randomised screening trial
Participants	53,454 people at high risk, enrolled at 33 US medical centres
Eligibility	Aged 55 to 74, at least 30 pack-years of smoking; former smokers had to have quit within the previous 15 years
Comparator	Three annual low-dose CT scans (26,722) against three annual single-view chest X-rays (26,732)
Primary endpoint	Death from lung cancer

Enrolment ran from August 2002 to April 2004, and outcomes were counted through the end of 2009. A pack-year is one pack a day for a year, so 30 pack-years is a substantial smoking history.

More than 90% of participants turned up for their screens, which is unusually high and worth remembering when reading the result.

The mortality result

There were 247 deaths from lung cancer per 100,000 person-years in the CT group and 309 per 100,000 person-years in the X-ray group. That is a relative reduction of 20.0% (95% CI 6.8 to 26.7, p=0.004).

Deaths from any cause were also lower with CT screening, by 6.7% (95% CI 1.2 to 13.6, p=0.02).

Relative reductions can mislead, so it helps to see the same finding in absolute terms. NCI puts it at roughly three fewer lung-cancer deaths per 1,000 people screened over about seven years of observation: 17.6 per 1,000 in the CT group against 20.7 per 1,000 in the X-ray group.

Twenty-four percent positive, and almost all of them wrong

Across the three rounds, 24.2% of CT screens came back positive, against 6.9% of chest X-rays.

96.4% of the positive CT results were false positives. For the X-rays it was 94.5%. So roughly one in four people screened by CT got a result that needed chasing, and the overwhelming majority of those chases ended in nothing.

Chasing a positive result is not free. It means more scans, sometimes a needle or a surgical biopsy, and months of not knowing.

More cancers found in the CT group

The CT arm turned up 1,060 lung cancers, at 645 per 100,000 person-years. The X-ray arm turned up 941, at 572 per 100,000 person-years. The rate ratio was 1.13 (95% CI 1.03 to 1.23).

Some of that excess is early detection of cancers that would have declared themselves later. Some of it may be overdiagnosis: cancers found that would never have caused symptoms or shortened a life. This trial cannot tell you the split, and that uncertainty is a permanent feature of screening evidence.

Not every tumour type benefited equally either. NCI notes that adenocarcinomas and squamous cell carcinomas were caught at the earliest stage more often by CT, while small-cell lung cancers — which move fast — were rarely caught early by either test.

What this trial cannot tell you

• What screening does for anyone outside the entry criteria. Lighter smokers, people under 55 or over 74, and never-smokers were not studied.
• How much of the extra cancer detection is overdiagnosis. The trial measures the excess, not its meaning.
• What happens in an ordinary clinic. Screening here was done at 33 experienced US centres with over 90% attendance.
• Whether it beats no screening at all. The comparison was chest X-ray, which earlier work had already found unhelpful for lung-cancer death rates.

Questions about lung screening

• Do I meet the criteria this trial used, and if not, how far off am I?
• What happens if my scan shows something, and how likely is that?
• How many years of annual scans are we talking about?

Sources

This article was written from the sources below, which were checked on the source-check date shown above.

• NCI: National Lung Screening Trial (NLST) (official)
• NEJM: Reduced Lung-Cancer Mortality with Low-Dose Computed Tomographic Screening (primary)
• NEJM full text: Reduced Lung-Cancer Mortality with Low-Dose CT Screening (primary)

How this article was prepared

Prepared by Cancer Explained's AI-assisted editorial system and checked against the sources listed below. This article has not been reviewed by a healthcare professional unless a named reviewer is specifically shown.

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