It may soon be possible to distinguish aggressive prostate tumors requiring immediate treatment from those that grow slowly and can be safely ignored, a problem that has vexed oncologists and patients for decades.

Looking at the complete profiles of chemicals produced by prostate tumors, researchers found that levels of sarcosine — a simple derivative of the amino acid glycine — are substantially higher in patients with aggressive tumors, they reported Wednesday in the journal Nature.

Levels of sarcosine in the blood could be easily and cheaply identified with a simple test that could replace or complement the prostate-specific antigen (PSA) test now widely used in prostate cancer patients. PSA is very useful for identifying the presence of tumors, but says little about their ultimate prognosis.

Perhaps equally important as the metabolic findings, tests in laboratory dishes showed that adding sarcosine to benign tumors dramatically increases their aggressiveness, while blocking sarcosine production in aggressive tumors renders them much less potent.

The finding may eventually lead to new ways of preventing the spread of the tumors, said Dr. Arul M. Chinnaiyan, a pathologist at the University of Michigan Medical School and senior author of the Nature paper.

Prostate cancer is the most common form of cancer in U.S. men, with an estimated 186,320 cases diagnosed last year, according to the American Cancer Society. About 28,660 died of the disease. Fully 80 percent of men over the age of 80 develop it.

The conundrum for clinicians is that while many of the tumors can be highly aggressive and immediately life-threatening, others are so slow-growing that they can be safely ignored, a concept known as watchful waiting. But there has not been any good way to distinguish between the two forms.

To address the problem, the Michigan group used the new science of metabolomics, in which researchers look at all the metabolites — chemicals produced during metabolism — associated with an organ. They studied 1,126 metabolites in 262 samples of healthy prostate tissue, localized prostate tumors and aggressive or metastatic prostate cancer.

The researchers identified 10 chemicals whose levels increased during prostate cancer progression. The most dramatic results were with sarcosine: Levels were elevated sharply in 79 percent of metastatic cancer patients and less sharply in 42 percent of the early stage patients. The chemical was not found at all in cancer-free samples.