A blood test may be able to tell doctors when melanoma patients are relapsing, according to a study published in the journal Cancer Discovery.

Melanoma, while the least common form of skin cancer, is the most deadly. The American Cancer Society says there will be 76,380 new cases of melanoma in 2016, and the incidence of melanoma has doubled since 1973.

Scientists at the Cancer Research UK Manchester Institute studied the DNA shed by melanoma tumors into the bloodstream, called circulating DNA.

They found that they could tell if a patient was beginning to relapse by tracking levels of circulating tumor DNA. They also discovered new mutations in genes that could possibly allow the tumor to become resistant to treatment and thus cause the relapse.

Melanoma patients usually respond to treatment when first diagnosed, but many melanomas become resistant to treatment within a year.

Up to 50 per cent of melanoma patients have a faulty BRAF gene, and although they can be treated with the targeted drugs vemurafenib or dabrafenib, either they don't work or the tumors quickly develop resistance.

In either case, these patients can be offered immunotherapy drugs including pembrolizumab, nivolumab and ipilimumab. But spotting a relapse early in the process by keeping tabs on circulating tumor DNA, doctors could tailor treatments for individual patients, resulting in improved care and increased chances of survival.

"Being able to spot the first signs of relapse, so we can rapidly decide the best treatment strategy, is an important area for research," says lead author Richard Marais. "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."

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," says Peter Johnson, Cancer Research UK's chief clinician. "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," Johnson said, "but we hope that in the future, blood tests like these will help us to stay one step ahead in treating cancer."