Key points from article :
Immunotherapy can be effective in some men with advanced prostate cancer.
The drug pembrolizumab caused the tumour to shrink or disappear in 1 in 20 with advanced disease.
Although this 5 % means a small number, some of them gained years of extra life.
Another 19% saw evidence of some sort of improvement.
This trial involved 258 men with advanced prostate cancer who didn't respond to other treatment options.
Those who have mutations in genes involved in repairing DNA in the tumour may benefit more.
Research by the Institute of Cancer Research and the Royal Marsden, published in Journal of Clinical Oncology.
