A major trial of an immunotherapy drug has shown it can be effective in some men with advanced prostate cancer.
The men had stopped responding to the main treatment options.
Researchers found that a small proportion of men, described as "super responders", remained well even after the trial ended, despite a very poor prognosis before treatment.
Last week it was reported the same drug had proved effective in treating advanced head and neck cancers.
What is immunotherapy?
Immunotherapy uses our own immune systems to recognise and attack cancer cells.
It's already being used as a standard treatment for some cancers such as melanomas - and being tested on many others too.
What did the study find?
It found that one in 20 men with advanced prostate cancer responded to the drug pembrolizumab - and saw their tumours actually shrink or disappear altogether.
Although a relatively small number, some of them gained years of extra life, the study in the Journal of Clinical Oncology found.
A further 19% saw some evidence of improvement.
But most patients in the study lived for an average of eight months on the drug.
The phase II clinical trial, led by the Institute of Cancer Research and the Royal Marsden, involved 258 men with advanced prostate cancer who had run out of all other options on treatment.
What happens next?
The most dramatic responses were seen in patients whose tumours had mutations in genes involved in repairing DNA.
Researchers are now investigating whether this group might benefit the most from immunotherapy in a larger trial.
But first, a test to pick out who will respond best is needed, so that doctors know which patients to give it to.
What is prostate cancer?
It's the most common cancer in men in the UK, with around 47,700 diagnosed in the UK each year.
The number of people diagnosed has been rising over the last 10 years.
This is probably because the population is getting older and more people are having PSA tests.
Around 30% of men with advanced or stage 4 prostate cancer survive their cancer for five years or more after diagnosis.
What do experts say?
"Immunotherapy has had tremendous benefits for some cancer patients, and it's fantastic news that even in prostate cancer, where we don't see much immune activity, a proportion of men are responding well to treatment," says Professor Paul Workman, chief executive of The Institute of Cancer Research.
"A limitation with immunotherapy is that there's no good test to pick out those who are most likely to respond.
"It's encouraging to see testing for DNA repair mutations may identify some patients who are more likely to respond, and I'm keen to see how the new, larger trial in this group of patients plays out."
Professor Johann de Bono, a consultant medical oncologist at The Royal Marsden NHS Foundation Trust, was one of those who led the study.
He said: "We don't see much activity from the immune system in prostate tumours, so many oncologists thought immunotherapy wouldn't work for this cancer type.
"But our study shows that a small proportion of men with end-stage cancer do respond, and crucially that some of these men do very well indeed.
"We found that men with mutations in DNA repair genes respond especially well to immunotherapy, including two of my own patients who have now been on the drug for more than two years."
Last week, a separate trial found the same drug kept some people's advanced head and neck cancers at bay for an average of two years - five times longer than under chemotherapy.
Both studies are part of a growing body of research suggesting immunotherapy could offer hope to an increasing number of cancer patients.