Scientists have the first major evidence that blood tests called liquid biopsies hold promise for screening people for cancer.
Hong Kong doctors have tried it for a type of head and neck cancer and boosted early detection and one measure of survival.
The tests detect DNA that tumours shed into the blood.
Some are used now to monitor cancer patients and many companies are trying to develop versions of these for screening, as possible alternatives to mammograms, colonoscopies and other such tests.
The new study shows this approach can work, at least for this one form of cancer and in a country where it's common.
"This work is very exciting on the larger scale" because it gives a blueprint for how to make tests for other tumour types such as lung or breast, said Dr Dennis Lo, of the Chinese University of Hong Kong.
He led the study, published on Wednesday by the New England Journal of Medicine.
Lo is best known for discovering that fetal DNA can be found in a mother's blood, which launched a new era of non-invasive testing for pregnant women.
The study involved nasopharyngeal cancer, which forms at the top of the throat behind the nose.
It is a good test case for DNA screening because it is an aggressive cancer for which early detection matters a lot.
About 20,000 men were screened, and viral DNA was found in 1112, or 5.5 per cent.
Of those, 309 also had the DNA on confirmatory tests a month later.
After endoscope and MRI exams, 34 turned out to have cancer.
More cases were found at the earliest stage - 71 per cent versus only 20 per cent of a comparison group of men who had been treated for nasopharyngeal cancer during the previous five years.
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