A trial of a prostate cancer drug at a London hospital has been stopped early because it was showing such good results that doctors felt it would be unethical to withhold it from the placebo group.

Radium-223 chloride, known as Alpharadin TM, targets tumours with high precision using alpha radiation, said the doctors conducting the study.

Dr. Chris Parker, lead researcher on the project at the Royal Marsden Hospital, said: "It's more damaging. It takes one, two, three hits to kill a cancer cell compared with thousands of hits for beta particles."

The Daily Telegraph reports the drug is seriously damaging to cancer cells while sparing surrounding tissue.

At an international forum of cancer experts, Parker said: "They have such a tiny range, a few millionths of a metre. So we can be sure that the damage is being done where it should be."

The study has so far shown that patients taking Alpharadin TM have a 30 per cent lower rate of death compared with patients taking the placebo.

"It would have been unethical not to offer the active treatment to those taking placebo," Parker said, adding that Alpharadin TM has "a completely different safety profile."

Professor Gillies McKenna, a radiology expert at Cancer Research UK, said: "This appears to be an important study using a highly targeted form of radiation to treat prostate cancer that has spread to the bones. ... This research looks very promising and could be an important addition to approaches available to treat secondary tumours - and should be investigated further."

Parker and his team will now submit their findings for approval by regulators.