Oestrogen skin patches used to control menopausal symptoms in women may provide a safe alternative therapy for prostate cancer, research has shown.
Hormone replacement therapy (HRT) patches prevent the growth of prostate tumours by drastically lowering levels of testosterone.
Prostate cancer that is starting to spread is known to be fuelled by the male hormone.
Current treatments include injected drugs called LHRH agonists, such as Zoladex, which interfere with testosterone production, resulting in chemical castration.
However, they can have serious long-term side effects, including the bone thinning disease osteoporosis and diabetes.
Oestrogen - the female sex hormone - taken in a pill form also blocks testosterone production, but can lead to dangerous blood clots and strokes.
The new findings, from a Phase II trial involving more than 250 men with locally advanced or spreading prostate cancer, show that the patches lower testosterone as effectively as LHRH agonists.
Importantly, they do not cause the same blood clotting problems seen with oestrogen tablets. Nor do they have the side effects associated with LHRH agonist injections.
The chief adverse effects of using oestrogen patches were swollen breasts and irritated skin.
Lead researcher Dr Ruth Langley, from the Medical Research Council Clinical Trials Unit in London, said: "These promising new findings suggest that we might be able to use oestrogen patches or an oestrogen gel to treat prostate cancer without significantly increasing the risk of heart disease and stroke.
"We think the reason oral oestrogen caused these side effects is because the oestrogen reached the liver in high concentrations straight from the stomach, whereas if the oestrogen can be absorbed through the skin the effect on the liver is avoided."
The study, funded by Cancer Research UK, appears in the latest edition of the medical journal The Lancet Oncology.
Co-author Professor Paul Abel, from Imperial College London, said: "The next step is to test if the oestrogen patches are as effective at stopping the growth of prostate cancer as the current hormone treatments.
"We're now testing this in over 600 patients and some early results could be available later this year."
Kate Law, director of clinical and population research at Cancer Research UK, said: "More men than ever are surviving prostate cancer thanks to advances in research, but we still urgently need to find more effective treatments and reduce side effects.
"This trial is an important step towards better and kinder treatments that could bring big benefits to men with prostate cancer in the future."