Prostate cancer kills 10,000 men in the UK each year
Researchers are probing an unexpected success in a study of an experimental treatment for prostate cancer.
In three men with advanced disease, use of an immune drug called ipilimumab, shrank their tumours to such an extent surgeons were able to operate.
The Mayo Clinic team in the US said the "startling" results in the study of 108 men had prompted them to set up a second trial using higher doses.
One UK expert said there were currently few treatments for advanced disease.
In men with advanced prostate cancer, which has spread outside the prostate, surgery cannot usually be done.
Hormone therapy is usually given to try to shrink the tumour to some degree and buy some time.
The trial was set up to see if MDX-010, a type of drug called a monoclonal antibody, would improve on hormone treatment.
The idea is that the drug will encourage a strong immune response to attack the cancer cells.
Half the men had normal therapy and half also received MDX-010.
In three cases, where the experimental drug was given, the tumours shrank dramatically, enabling surgeons to operate and remove the tumour.
There are 20 other patients who are showing improvements and who are being monitored by the surgeons.
Dr Eugene Kwon, a surgeon at the Mayo Clinic in Rochester, said the results in those men were well beyond their expectations.
"Our surgeons had never seen this happen before and we were really taken by surprise."
They are now planning a trial in 30 men to test higher doses of the drug and hope to start much larger trials across many hospitals shortly after.
Dr Michael Blute, study leader and the surgeon involved said: "I had never seen anything like this before. I had a hard time finding the cancer.
"At one point the pathologist (who was working during surgery) asked if we were sending him samples from the same patient."
Until large scale studies are carried out it is unclear whether this response can be repeated in other patients or is an anomaly.
But John Neate, chief executive of The Prostate Cancer Charity said they would wait for further results with anticipation.
"If a cancerous tumour has spread beyond the prostate gland, it would currently be regarded as inoperable and alternative types of treatment, typically hormone therapy are necessary.
"If these early and small scale results are replicated in larger trials, this represents a potentially very exciting development.
"We urgently need a wider range of treatment options for prostate cancer which has spread outside the prostate gland.
"It must be remembered that this is a small trial however, and the findings are preliminary."