Researchers say it is so sensitive it can even differentiate between patients whose tumours are at the same stage.

If this test were done on tumours of newly diagnosed patients, we would have an indication of which cancers were most aggressive

Professor Akhouri Sinha

At present the degree to which prostate cancers have progressed is determined by the Gleason grading system, which is based on the shape and microscopic appearance of tumours.

Patients with higher grades of tumour are in more advanced stages of the disease, but the grade does not tell how aggressively the tumours have been growing or stimulating the development of secondary growths.

Prostate cancer is set to become the most common male cancer by 2006, as lung cancer rates fall and the population ages.

On the increase

Incidence of the cancer has been rising steadily over the last 30 years.

It now affects around 22,000 men in the UK each year and is fatal for around 10,000.

It could aid physicians in determining what level of treatment would be best for a given patient

Dr Charlotte Bevan

The Gleason grading system assigns tumours a score between two and 10, with 10 being the most advanced cancer.

Patients with a Gleason score from seven to 10 have a higher risk of dying of prostate cancer than those with lower scores, but some patients with higher scores outlive some with lower scores.

This means that within any score, there are biologically aggressive and less aggressive forms of cancer.

This makes it more difficult to predict outcomes for individual patients.

Enzyme clue

The new test, developed by Professor Akhouri Sinha, of the University of Minnesota, works by measuring concentrations of two important chemicals.

One, an enzyme called cathepsin B (CB), helps cancer cells to invade surrounding healthy tissue.

The other, called stefin A, blocks the action of cathepsin B.

The researchers analysed tissue samples from 97 prostate cancer patients and eight patients with a benign enlargement of the prostate.

They found that the level of cathepsin B relative to the level of Stefin A was significantly higher in patients whose cancer had spread.

Professor Sinha said: "The ratio of CB to stefin A reveals differences in tumours that are not visible under the microscope.

"If this test were done on tumours of newly diagnosed patients, we would have an indication of which cancers were most aggressive, and we could give those patients aggressive treatment.

Professor Sinha said the same approach could also be used for breast and colon cancer.

Reaction

Dr Charlotte Bevan, of the Prostate Cancer Charity, said: "This is a very exciting development as it could aid physicians in determining what level of treatment would be best for a given patient.

"As cancer therapy itself can be very aggressive and exhausting for a patient and produce undesirable side-effects, any test that identifies cases where therapy can be minimised or made less aggressive is going to significantly improve the lives of these patients and their friends and families."

Dr Sara Hiom, a science information officer at the charity Cancer Research UK, said scientists around the world were trying to find a way to pinpoint which prostate cancer tumours were potentially fatal, and which did not require intensive treatment.

"We would welcome any accurate test that could distinguish between aggressive and indolent types of prostate cancer.

"We await results of further studies with great interest."

UK scientists from the Institute of Cancer Research recently announced a major initiative designed to find ways to differentiate between prostate tumours.

Dr Chris Parker told BBC News Online that the US study showed promise in one particular field, that of identifying molecular differences between tumours.

The UK effort will also focus on differences in the blood supply and oxygen supply between tumours as well as testing thousands of potential molecular markers.

The research is published in the journal Cancer.