What makes the decision even more difficult is that doctors cannot always tell which cancers will grow and become life-threatening, and which will develop slowly over decades without causing any symptoms.

British doctors who are pioneering high intensity focused ultrasound (HIFU) believe that the technique offers men a less risky alternative.

It uses sound waves generated from a probe inserted in the rectum which are focused into an area of the prostate the size of a rice grain.

First, the probe scans the prostate and displays a three-dimensional image of the gland on a computer screen. The surgeon then draws on the screen to tell the computer which areas of the prostate to destroy and which nerves to avoid. The probe heats up each piece of tissue to 90C in just one second, melting fat within cells and causing them to die.

The operation is over in less than two and a half hours and is usually done under an epidural or regional anaesthetic.

Surgeons can steer the beam away from the neurovascular nerves that are needed to control erections and they can also avoid damaging the rectum and urinary sphincter.

High intensity ultrasound destroys the part of the urethra passing through the prostate. So doctors insert a small catheter to connect the bladder to the end of the penis. After a few days, tissue grows around this tube, which can then be removed.

Mark Emberton, the surgeon who is carrying out the trial at the Institute of Urology, University College London, will test the technique on 150 men with early to medium stage prostate cancer, with prostate specific antigen levels less than 15 and a prostate that is less than 40cc in volume. Most will be aged between 50 and 79.

Patients will also be recruited by the surgeon, Simon Brewster, at Churchill Hospital, Oxford.

They will be treated once and then monitored for five years. The trial will look at side-effects and efficacy.

After surgery or radiotherapy, 80 to 90 per cent of men with this stage of prostate cancer should be alive and free of disease, he said. The HIFU team hope their approach will be even more successful.

Doctors have been interested in high intensity focused ultrasound for more than 50 years. However, it has only been in the last few years that the technique has been used to treat cancers. Chinese doctors pioneered work on high intensity focused ultrasound and have carried out trials on nearly 5,000 people.

According to Cancer Research UK, it could be used for cancers of the pancreas, bladder, kidney and liver as well as the prostate.

High intensity focused ultrasound research in France and America includes work on brain tumours and other cancers of the head and neck. The skull is entered through positioning the probe at the temporal lobes.

A British trial is investigating high intensity focused ultrasound for liver and kidney cancer at Churchill Hospital.

Patients are being treated with ultrasound and then a week or two later their cancers are removed by surgery.

The National Institute for Clinical Excellence is currently compiling advice to the NHS on ultrasound and other prostate cancer treatments.

The evidence has been mixed although most studies have shown it is as effective as conventional treatments.

A Japanese study at Tokai University Hachioji Hospital looked at its effect on 132 prostate cancer patients. After one year, 83 per cent were free from disease while, after five years, 69 per cent were disease-free. Around one fifth complained of impotence after treatment compared with two-thirds of surgery patients.

Nick Stevens, of UKHIFU, believes the new technique could change attitudes towards prostate screening.

"All of the current treatments involve harming the patient," he said. "In the weighing up of risks and benefit of treatment, policy makers have said the balance has been in favour of not creating unnecessary harm.

"High intensity focused ultrasound possibly offers a primary care treatment for prostate cancer. It means all men screened between 55 and 65 could have a treatment which doesn't cause high levels of impotence and incontinence, or involve waking up in a hospital bed or 40 visits for radiotherapy."

For more information about the trial see www.ukhifu.co.uk or call 0845 4567853.

The Prostate Cancer Charity's confidential helpline 0845 300 8383 is staffed by trained nurses and open from 10am to 4pm Monday to Friday and Wednesdays from 7-9pm.

Prostate cancer is the most common form of cancer in men. Each year, 30,000 sufferers are diagnosed and around 10,000 die.