WASHINGTON - Doctors looking to see if cancer has spread may be able to simply listen for it in the future, U.S researchers reported on Tuesday.

Researchers at the University of Missouri-Columbia said they used a technique called photoacoustic detection to pick up the characteristic vibrations of melanoma cells in the blood.

They said their method could let oncologists spot as few as 10 cancer cells in a blood sample, catching a tumor's spread before it can settle into another organ.

Writing in the journal Optics Letters, the University of Missouri team said they combined laser techniques from optics and ultrasound techniques from acoustics, using a laser to make cells vibrate and then picking up the characteristic sound of melanoma cells.

They said they were able to detect melanoma cells obtained from actual patients.

The dark, microscopic granules of melanin in the melanoma cancer cells absorb the energy bursts from the blue-laser light. As the melanoma cells expand and contract, they generate crackling sounds that can be picked them up with special microphones and analyzed by computer.

Other human cells do not contain pigments with the same color as melanin, so the melanin signature is easy to tell apart from other noises, said John Viator, a biomedical engineer who worked on the study.

"The only reason there could be melanin in the human blood is that there would be melanoma cells," he said.

A blood screening test could reassure patients who have a growth removed -- or tell a doctor to start chemotherapy quickly because the cancer has already started to spread.

"It could take just 30 minutes to find out if there are any circulating cancer cells," Viator said.

Melanoma is the deadliest form of skin cancer and spreads quickly if not removed promptly. The American Cancer Society estimates that in 2006 there will be 62,190 new cases of melanoma in the United States alone and 7,910 deaths.

Because of melanin, melanoma is the only type of cancer that can be detected in this way. But the researchers said they could try using artificial materials to act as light absorbers and as noise makers.

"We're looking for methods to attach other kinds of absorbers to cancer cells," Viator said.