The period after cancer surgery can be a stressful and uncertain time.

Despite surgeons’ best efforts, it’s difficult to know whether someone’s cancer has been completely removed, or if a little is left behind. And that distinction is crucial, because even if a small number of cancer cells remain, it could mean a patient’s cancer comes back.

To try and stop this, many patients will have additional therapy after surgery. It’s the most proactive approach, but it has its downsides. Inevitably, there will be people who have actually been cured by surgery that will have unnecessary, and often toxic, treatment.

But the alternative – to monitor for signs of cancer after surgery – isn’t ideal either. If someone does have residual cancer, they may have to wait until their disease can be seen on a scan before receiving potentially beneficial treatment.

It’s a problem researchers like Professor Thomas Powles at the Cancer Research UK Barts Centre are trying to solve. And they might have landed on an answer, in the form of a blood test.

Figuring out who would benefit from additional treatment

“Giving patients personalised treatment based on blood tests, rather than treating all patients the same or waiting to see if patients relapse, would be a big – and needed – change in how we currently treat cancer,” said Powles.

To see if it was possible, Powles and colleagues set up a trial testing whether a blood test could tell whether bladder cancer patients would benefit from further treatment after surgery.

Cancer blood tests

Scientists are looking at a number of different ways to pick up signs of cancer in the blood – from entire cancer cells to tiny fragments. This includes DNA shed by cancer cells into the bloodstream, called circulating tumour DNA (ctDNA).

The tests can tell the difference between ctDNA and the DNA from healthy cells as they look for certain changes in the ctDNA unique to the tumour it came from. As patients have their tumour DNA sequenced after surgery, scientists can match the DNA in the blood to the tumour.

If patients test positive for ctDNA in their blood, they’re more likely to have their disease come back.

The study, published in the journal Nature, involved over 500 bladder cancer patients who had received surgery to remove their tumours, randomly assigning them into one of two groups. One of these groups was monitored after surgery, whilst the other received an immunotherapy drug called atezolizumab.

All patients had blood tests to see if signs of cancer could be found in their bloodstream.

Encouragingly, atezolizumab reduced the risk of a patient’s cancer coming back or dying from cancer by around 40% if tumour DNA could be found in their blood. The treatment also cleared ctDNA from the blood of around 1 in 5 patients.

Atezolizumab made no difference for patients who were negative for ctDNA. This shows that a blood test may be used to tell which patients would benefit from this treatment, whilst sparing those who don’t need it.

Previous studies have shown that blood tests after surgery can be used to indicate how well patients will do, but this is one of the first to show that they can also be used to select patients for additional treatment.

Catching cancer when it’s more treatable

Blood tests aren’t only useful for telling who would benefit from certain treatments, but can also detect signs of cancer returning earlier than scans.

Treating patients as soon as there are signs of their cancer returning gives them a better chance of surviving their disease.

“As cancer grows, it gets more biologically complex. As it gets more complicated, gaining new errors in its DNA, it gets harder to treat,” says Powles.

“Catching cancer before it’s even detectable on scans gives us a ‘magic window’ where we’ve not given it the chance to become resistant to treatments, and means it’s much more likely to be cured.”

Blood tests aren’t a silver bullet though.

This study found that some people who tested negative for ctDNA still relapsed, and Powles also points out that we know that not every type of cancer sheds DNA into the bloodstream. But as these tests develop, they have the potential to transform the way a lot of patients are treated.

This test is now going into further clinical trials to validate these results. This includes one that will monitor patients for a long time after surgery to see if blood tests could be used to replace the regular follow up scans that patients usually have to go through.

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