A universal blood screening test that detects cancer years before symptoms appear could revolutionise treatment, scientists say.
Researchers attending the world's biggest cancer conference in Chicago unveiled studies showing that a blood test accurately picks up mutations for several different types of cancer.
Work is now under way, backed by Microsoft founder and billionaire Bill Gates, to use this knowledge to develop a screening test that can find the earliest signs of cancer in people who appear healthy.
The hope is that people could have the test in their GP surgery as part of an annual health check, alongside monitoring of their blood pressure or cholesterol.
Those that are found to have cancer could then have surgery or other treatment. The move could potentially prevent thousands of deaths per year from advanced cancer.
The US firm Grail, which is backed by 100m dollars (£77.5m) of funding from Gates and Amazon chief Jeff Bezos, has set a goal of introducing such a test by 2019.
The technique could also be used to spare people with cancer from needing to undergo painful biopsies of their tumour.
The test is based on the knowledge that all cells in the body release information into the bloodstream through secretion or as they die.
Cancer tumours are also known to shed this information - known as circulating tumour DNA (ctDNA).
By analysing ctDNA, experts are already able to tailor treatments for people with advanced cancer. They also know that larger amounts of ctDNA in the blood signify more advanced disease.
But, in a new step, they are now working to harness this knowledge to create a test specifically for early-stage disease.
Dr Nicholas Turner, from the Institute of Cancer Research (ICR) in London and the Royal Marsden, said much more work was needed to perfect the test but there was excitement about the prospect.
He said: "The potential is absolutely clear and the potential is very exciting - that you might be able to screen patients to detect whether they've got cancer by looking for traces of cancer in the blood.
"The potential is dramatic if we could identify patients with, say, pancreatic or lung cancer at the point they could all have surgery, you could potentially transform management of the disease and survival for patients.
"The cancer community is extremely excited about the potential but whether that potential can be achieved is not clear enough yet."
He said one issue to overcome was that of the test returning some false positives - where people are thought to have cancer when they do not.
For example, as people get older, their cells naturally shed genetic mutation information into the blood but not all these people will develop cancer.
He said: "Working out how you can develop assays that accurately predict who's got cancer and who will go on to get cancer is a real challenge.
"It's clearly going to be many years until the assays come through but a large number of biotech firms are devoting a substantial amount of money so I think we should be very optimistic that they will come through and the challenge of false positives will be overcome."
He said cancers such as pancreatic, ovarian and lung are difficult to pick up in the early stages because symptoms are vague.
Even if a test could just pick up half of these cases, that would be a "substantial advance on where we are currently", he said.
One new study presented at the American Society of Clinical Oncology (ASCO) meeting in Chicago was on 124 patients with advanced breast, lung, and prostate cancers.
In 89% of patients, at least one genetic change detected in the tumour was also detected in ctDNA in the blood.
The test scanned a very broad area of the genome (508 genes and more than two million base pairs or letters of the genome) with high accuracy.
Lead study author Pedram Razavi, who received funding from Grail and is from the Memorial Sloan Kettering Cancer Centre (MSK) in New York, said the test was the most advanced to date and the results of the study were "very promising".
He added: "This study is also an important step in the process of developing blood tests for early detection of cancer."
In a second study presented at ASCO on 199 patients with pancreatic cancer, detectable ctDNA was found in 56% of them.
This included 43% of those with the earliest stage of disease.
Peter Gibbs, associate professor of medical oncology at the Royal Melbourne and Western Hospitals in Australia, who worked on the study, said he thought an early cancer blood test was three to five years away.
He added: "You'd have your cholesterol check, your blood sugar check and DNA check at the same time. That's what I would envisage happening."
Bert Vogelstein, professor of oncology and pathology at Johns Hopkins University School of Medicine in Baltimore, said an early detection test could bring survival rates from all types of cancer to "well over 90%".
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