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.
Prof de Bono added: "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."
But the ICR has warned that the treatment will still only work for a minority of patients, with only around 20% likely to respond to immunotherapy.
Research will now focus on identifying signs to help pick out the prostate cancer patients whose tumours are most likely to shrink after the treatment.