US Researchers have developed a test which can detect the spread of melanoma, a form of skin cancer, by listening out for the presence of cancerous cells in the blood. The technique, known as photoacoustic detection, is sensitive enough to pick up just ten cancer cells in a blood sample containing millions of cells. Writing in the journal Optics Letters, John Viator and his colleagues at the University of Missouri-Columbia developed the approach using cultured melanoma cells from a cancer patient. But they also outline how the test could be used as an early warning system to pick up cancer spread in its earliest stages. They suggest collecting blood from potential cancer cases. The red blood cells and plasma would then be discarded, leaving behind just the white blood cells and any melanoma cells. These would then be illuminated with brief (five billionths of a second) bursts of blue light from a laser. Melanoma cells preferentially absorb the laser energy because they contain granules of the brown melanin pigment. This causes them to expand and contract with each flash of the laser, producing ultrasonic shockwaves that can be picked up with a specialised microphone. As other human cells don't contain pigments the same colour as melanin, the melanin "signature" is easy to spot. And the presence of such a melanin signal is highly diagnostic. "The only reason there could be melanin in the human blood is that there would be melanoma cells," explains Viator.
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