A pioneering new drug appears to have cured a British man with advanced skin cancer who had been given months to live.
Doctors cannot be certain it was the treatment that led to the miraculous outcome, but know of no other explanation.
Results from an early-stage trial of the drug indicate that it may offer a potential "paradigm shift" in cancer therapy, according to the patient's consultant.
The drug, pembrolizumab, is the latest in a new generation of treatments that prevent cancers shielding themselves from the immune system.
It was tested on melanoma because the prospects for patients with advanced forms of this disease are so bleak.
Just under 70% of the 411 patients taking part in the trial were still alive one year after starting on the treatment. The result is considered remarkable because all had highly advanced melanoma and a very poor prognosis.
Currently one-year survival rates for untreated patients diagnosed with advanced Stage Four melanoma are just 10% for men and 35% for women. The "cured" trial patient, a 40-year-old London man, has undergone six months of treatment with pembrolizumab.
Doctors were astonished when after just three months his tumours had almost disappeared. Since then they have shown no sign of returning – and in fact have shrunk even further.
His consultant Dr David Chao, from the Royal Free Hampstead NHS Trust in London, said: "We cannot say for certain he's been cured, but he is doing very well."
Results from the trial were presented at the annual meeting of the American Society of Clinical Oncology in Chicago.