After being given only a single dose of an experimental drug, called ipilimumab, two patients with inoperable prostate cancer have made remarkable recoveries.
Rodger Nelson and Fructuoso Solano-Revuelta were diagnosed with advanced prostate cancer, and were part of a trial at the Mayo Clinic in Minnesota in the US that involved 108 patients, half of whom received the experimental drug.
The drug's results were so impressive that researchers decided to release details of the two cases before the drug trial was complete.
Dr Eugene Kwon, the urologist who was in charge of their treatment, said that their progress had exceeded all expectations, and likened the results to the first pilot breaking the sound barrier.
"This is one of the Holy Grails of prostate cancer research. We have been looking for this for years," the Independent quoted him as saying.
Initially, the pair were told that the disease had spread beyond the prostate. Nelson's cancer was encroaching on the abdomen and Solano-Revuelta's tumor was the size of a golf ball.
Usually, patients in such condition are told they may have only months to live, and are normally only offered palliative care.
But after one infusion of the drug ipilimumab, a monoclonal antibody that stimulates the immune system, given with conventional hormone therapy, their tumors shrank enough to be surgically removed.
Since then, both men have made a full recovery and returned to their businesses.
Although the trial is still going on, but the improvement of the two patients was so dramatic and unexpected that they were removed from the study so they could undergo curative surgery.
"Halfway through the trial we began seeing remarkable responses. Some patients had dramatic shrinkage of their tumors so practically all traces had disappeared. We had thought we might get some incremental delay in the progression of the cancer. It had not dawned on us that we might go from an inoperable tumor to an operable one. That just doesn't happen," said Kwon.