A new blood test could prove to be a lifeline for people with advanced melanoma, the most dangerous form of skin cancer.
By spotting tumour DNA in the bloodstream, the test can give early warning of a disease that is becoming resistant to treatment.
New treatments can then be offered sooner, improving a patient's chances of survival.
Professor Richard Marais, from the Cancer Research UK Manchester Institute, said: "Being able to spot the first signs of relapse, so we can rapidly decide the best treatment strategy, is an important area for research.
"Using our technique, we hope that one day we will be able to spot when a patient's disease is coming back at the earliest point and start treatment against this much sooner, hopefully giving patients more time with their loved ones.
"Our work has identified a way for us to do this but we still need to test the approach in further clinical trials before it reaches patients in the clinic."
The scientists found a way to identify new genetic mutations linked to melanoma resistance by tracking circulating DNA.
Up to half of melanoma patients can successfully be treated with targeted drugs, but for many the therapies do not work or become ineffective after a relatively short time.
When the disease no longer responds to standard treatment patients can be offered advanced immunotherapy drugs, such as ipilimumab.
Professor Peter Johnson, Cancer Research UK's chief clinician, said: "One of the sinister things about melanoma is that it can lay dormant for years and then suddenly re-emerge, probably as it escapes from the control of the body's immune system.
"Being able to track cancers in real time as they evolve following treatment has huge potential for the way we monitor cancers and intervene to stop them growing back.
"There's still some time until we see this in the clinic but we hope that in the future, blood tests like these will help us to stay one step ahead in treating cancer."
The research, published in the journal Cancer Discovery, was funded by Cancer Research UK and the Wellcome Trust.
