WASHINGTON (Reuters) - Researchers believe they have found a compound produced by aggressive prostate tumours and said they hope they can use it to design a urine test to detect the most dangerous tumours.

Prostate cancer, which is found in 780,000 men every year globally and kills 250,000, can be very slow-growing. While common, it does not always require immediate treatment.

The difficult part is knowing which patients need the most careful monitoring after surgery or radiation therapy to destroy the tumour.

Scott Tomlins and colleagues at the University of Michigan Health System looked at 1,800 different prostate tumours to try to identify the unique "signature" of an aggressive type of prostate cancer found in about 10 percent of cases.

A gene called SPINK1 was over-expressed, or over-active, in about 44 percent of cases of prostate cancer, they reported in the journal Cancer Cell. Their findings suggested that cancer was twice as likely to come back after surgery among men with SPINK1-related prostate cancers.

Tumours come back after surgery in somewhere between 25 percent and 40 percent of men, they said.

"Because SPINK1 can be found non-invasively in urine, a test could be developed that would complement current urine testing that is used to detect some prostate cancer," Dr. Arul Chinnaiyan, who oversaw the research, said in a statement.

Tests for prostate cancer include a blood test that looks for prostate-specific antigen, or PSA — a protein produced only by prostate cells. Prostate tumours produce extra levels of PSA, but so does a prostate that is simply enlarging with age, making the test an imprecise one.

Doctors also use a digital rectal examination — using a finger to find an enlarged or lumpy prostate. San Diego-based Gen-Probe Inc. has a urine test to screen for a protein called PCA3, although it is not approved in the United States.

The University of Michigan has filed for a patent on SPINK1 as a biomarker of prostate cancer and licensed rights to Gen-Probe.

The researchers said they do not know precisely how SPINK1 is involved in cancer and will continue to investigate.