Two men being treated with an experimental antibody drug have seen their advanced and inoperable prostate cancer reversed.
Both men, who are now free of cancer, were taking part in the trial of drug called ipilimumab, designed to boost the immune system, at the Mayo Clinic in Rochester, Minnesota, USA.
Before the trial, both had aggressive tumours, which had progressed beyond the prostate and into abdominal areas.
As part of the trial, the men received traditional hormone therapy to remove testosterone, which fuels prostate cancer, then a single dose of ipilimumab.
The men saw their prostate specific antigen (PSA) levels drop to a level which made them eligible for surgery and tests showed the "tumours had shrunk dramatically".
After surgery one of the men underwent radiation treatment, but both have staged dramatic recoveries and have returned to normal life.
Mayo clinic urologist, Dr Michael Blute, said: "I had never seen anything like this before. I had a hard time finding the cancer. At one point the pathologist asked if we were sending him samples from the same patient."
Trial leader Dr Eugene Kwon added: "This is one of the holy grails of prostate cancer research. We've been looking for this for years."