Tumors release cells into the blood that can reveal that the cancer is growing and spreading to other parts of the body. Now, researchers have shown they can train a laser device on the hand of a skin cancer patient and detect these scarce tumor cells coursing through the bloodstream. The device could one day improve screening for the cancer melanoma. It could also help doctors monitor whether treatments are working, and even curb the original tumor’s spread by zapping the roaming cells.

“It’s fascinating that it’s possible to detect these circulating tumor cells literally through the skin,” says medical oncologist Klaus Pantel of the University Medical Center Hamburg-Eppendorf in Germany, who was not involved in the study. But he and others caution that much work remains to show the device will help people with melanoma.

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