A man with advanced treatment-resistant prostate cancer may have been "cured" by an experimental testosterone hormone therapy.
The patient's levels of Prostate Specific Antigen (PSA), a blood marker used to monitor prostate cancer, fell to zero after the therapy, which involves shocking tumours to death with testosterone.
The man showed no remaining trace of prostate cancer after 22 cycles of treatment, astounding US scientists who conducted a trial involving 47 participants.
Levels of PSA fell in the majority of patients involved in the study. All the male participants completed at least three cycles of "bipolar androgen therapy", which involves alternately flooding and starving the body of testosterone.
The treatment is groundbreaking because testosterone is generally assumed to fuel prostate cancer.
For decades, men with advanced and spreading prostate cancer have been treated by cutting off the supply of testosterone or blocking its effects.
However, the latest study suggests that blasting tumours with high levels of the hormone can suppress or even kill prostate cancer cells.
Lead researcher Professor Sam Denmeade, of Johns Hopkins University School of Medicine in Baltimore, said: "We think the results are unexpected and exciting.
"We are still in the early stages of figuring out how this works and how to incorporate it into the treatment paradigm for prostate cancer."
He added: "Thus far we have observed dramatic PSA response in a subset of men; PSA levels declined in about 40% of men and in about 30% of men levels fell by more than 50%.
"I think we may have cured one man whose PSA dropped to zero after three months and has remained so now for 22 cycles. His disease has all disappeared."
Prof Denmeade cautioned, however, that the treatment is still highly experimental and only suitable for men not battling painful symptoms.
"Testosterone treatment can definitely worsen pain in men with prostate cancer who have pain from their disease," he said.
Dr Matt Hobbs, deputy director of research at the charity Prostate Cancer UK, commented: "Many exciting new lines of attack against prostate cancer are emerging of which this is one.
"However, this is early stage research and further studies are needed in order to understand exactly how intriguing developments like this work and to test the findings more robustly in large clinical trials."
Each year around 47,000 men are diagnosed with prostate cancer in the UK and 11,000 die from the disease.