Prostate cancer patients with weeks or months 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 (ASCO) 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 38% were still alive after a year and 11% did not see the cancer grow, the results show.
Professor de Bono said: "In the last few years immunotherapy has changed the way we treat many advanced cancers - but up to now no one had demonstrated a benefit in men with prostate cancer.
"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."
He added: "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 5% 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.
Professor de Bono said: "We are planning a new clinical trial, specifically in men with prostate cancer whose tumours have mutations in DNA repair genes, to see if immunotherapy can become a standard part of their treatment.
"It's exciting that immunotherapy could offer some men more time with their loved ones where they have such advanced disease that they have run out of existing treatment options."
Only around 20% of cancer patients respond to immunotherapy and researchers do not fully understand why.
Professor Paul Workman, chief executive of the Institute of Cancer Research in London, said: "Immunotherapy has proven to be a smarter, kinder treatment for many types of cancer - but it still only works for a minority of patients.
"The challenges we now face are how to predict in advance who will benefit, and how to make immunotherapy work for more people.
"One of the major challenges with immunotherapy is that we don't have many reliable tests to pick out who will benefit.
"This new trial has found that testing for mutations in DNA repair genes could be a valuable marker of who will respond.
"If we can prove that in the planned new trial, it should be possible to provide some men with advanced prostate cancer with an exciting new treatment option."
Research will now focus on identifying markers to help pick out the prostate cancer patients whose tumours are most likely to shrink after immunotherapy treatment.