Prostate cancer patients with weeks or months left to live could survive longer after undergoing immunotherapy treatment, a major trial has shown.
More than a third of men with a very advanced form of the cancer were still alive, and one in 10 had not seen the cancer grow, after a year on the drug pembrolizumab, the study found.
It is the first time immunotherapy has been shown to benefit some men with prostate cancer, the researchers said.
The results of the trial, led by a team at the Institute of Cancer Research, London, and the Royal Marsden NHS Foundation Trust, will be presented at the American Society of Clinical Oncology annual meeting in Chicago.
Professor Johann de Bono, director of the drug development unit, said: "I have these men who are basically dying, with weeks to months to live, who we gave this drug to and had complete responses. Their cancers shrunk, disappeared actually, with minimal cancer left on scans."
He added: "These are amazing results, and these are men whose cancers had all the treatments, they had everything possible, they've got no treatments left, and they are in trouble.
"They have very short life spans left."
Immunotherapy drugs work by stimulating the immune system to recognise and fight the cancer, and are used to treat some advanced cancers, including lung and melanoma.
The trial of 258 men with advanced prostate cancer found they lived much longer when treated with "checkpoint inhibitor" pembrolizumab.
Around 38pc were still alive after a year and 11pc did not see the cancer grow, the results show.
Professor de Bono said: "Our study has found that immunotherapy can benefit a subset of men with advanced, otherwise untreatable prostate cancer, and these are most likely to include patients who have specific DNA repair mutations within their tumours.
"This will be another arrow to the quiver for a subset of prostate cancers and the results are preliminary but very promising."
Previous trials using immunotherapy in prostate cancer have been unsuccessful but the latest research examined the genetics of the tumours and found particular groups of patients may benefit.
While only 5pc of men in the trial saw their tumours shrink or disappear after treatment, many of those had mutations in genes involved in repairing DNA in their tumours.
The researchers suggest these mutating cancer cells may be easy for the immune system to recognise and attack because they look different from healthy cells.
Data from some other cancer types, such as bowel, has similarly shown tumours with defects in DNA repair mutations are more susceptible to immunotherapy.
Only around 20pc of cancer patients respond to immunotherapy and researchers do not fully understand why.