How does 'holy grail' cancer test work and when will it be available?

A type of early detection blood test for 10 different types of cancer has been hailed as a “holy grail” by researchers, with experts claiming such “liquid biopsies” could save lives. But how does it work, and when might it be rolled out?

Why do we need early detection of cancers?

Generally speaking, the earlier cancer is detected, the earlier treatment can begin and the better the outlook. The problem is that many types of cancer are tricky to spot in their early stages. Dr Justine Alford of Cancer Research UK used a hypothetical example of a patient with a brain tumour. “They don’t actually start to develop symptoms, which are really quite severe, until the brain tumour has started obstructing the way they can think and move,” she said. What’s more, in some cases invasive procedures such as endoscopies are needed to diagnose the cancer. A blood test would be less invasive and could be used to screen for cancer before symptoms appear.

How do liquid biopsies work – are there different mechanisms?

There are a number of different research groups working on liquid biopsies, and not all are hunting for the same clues in blood samples: some are looking for tell-tale proteins, others for tumour cells themselves, and some for small bits of genetic material.

The latest study, as yet unpublished, involved picking up on free-floating DNA in the blood released by cells, including tumours, possibly as a result of cell death. In a nutshell, the team took blood samples from people with and without cancer and looked at a number of different types of changes or mutations in free-floating DNA to see if they could accurately pick up on the presence of tumours in the body.

While the latest test only appears to flag whether someone has cancer or not, other liquid biopsies have combined different types of analysis to help hone in on where in the body the cancer is present.

Why do some cancers seem easier to detect with this test than others?

“We need to see the results of this and other studies to know that, but certain cancers seem to release more DNA into blood,” said Jacqui Shaw, professor of translational cancer at the University of Leicester, who is also working on liquid biopsies.

The latest study accurately spotted cancer in 90% of ovarian cases (although with a small sample size of 10 people) but picked up only 66% of the 28 cases of colorectal cancer.

Alford said there might be another factor at play: “It might be this is quite an early stage and so far they have not picked up all of the right molecules for all of the right cancers yet and that, so far, the mutations that they are looking at are perhaps more common in one type of cancer than another.”

Could liquid biopsies help to guide treatments as well?

That’s one of the hopes. A recent study in lung cancer patients has shown that the tests might help to pick up cancer recurrence far sooner than CT scans, while experts say they could help monitor whether treatments are having an effect in tackling tumours, and help patients get the best medication.

When might this be available on the NHS?

Not for several years. There are still many questions that need to be answered, and the issue of costs needs to be negotiated. One question is whether the tests could lead to false positives and unnecessary treatment of people without cancer, and whether they can tell a harmful tumour from a harmless one.

“A test like this [latest one] would have to go under years and years of observation and further testing to prove that it doesn’t actually cause more harm than good,” said Alford, who says reports it could be available in five years are overambitious.

Alford said it has not yet been shown that the approach will save lives. “At the moment [scientists] haven’t proven this test can pick up cancers that haven’t already been diagnosed; that is the really important thing that we need to stress here,” she said.

But many are excited. Simon Stevens, the chief executive of NHS England, said blood tests like that in the new study could “unlock enormous survival gains, as well as dramatic productivity benefits in the practice of medicine”.