Michael English, 72, was one of 258 men who took part in the trial.
He was first diagnosed in 2005, but radiotherapy, chemotherapy and hormone-based therapies did not kill his cancer.
Two years ago, he was given the immunotherapy drug pembrolizumab.
He said: "We were astonished when scans showed that the tumour had become undetectable.
"Today I'm effectively cancer-free."
He says he's now planning out the next 20 years of his life, not the next two.
Researcher Prof Johann de Bono told the BBC: "This is the first evidence that a subset of prostate cancer patients do spectacularly well on immunotherapy.
"We have several patients in the Marsden who have had a complete response.
"It is a new arrow in the quiver for men with lethal prostate cancer, it's a big deal for these patients."
(Excerpt) Read more at bbc.com ...
