WASHINGTON, March 15 (Xinhua) -- American scientists have developed a tool to test circulating tumor cells in blood vessels, which have long been feared as harbingers of metastasizing cancer but difficult to nab since they are drowned out by billions of ordinary blood cells.

In the study published on Thursday in the journal Clinical Cancer Research, a team led by Seungpyo Hong from the University of Wisconsin-Madison, has isolated those elusive tumor cells on clinical samples for the first time.

Researchers managed to slow down tumor cells, inspired by their behaviors to attach themselves to blood vessel walls, looking for places to invade, unlike the floating oxygen-carrying cells.

They used an array of sticky proteins to force the tumor cells to begin rolling, which slows them down.

The cells are then trapped using a series of three cancer-specific antibodies, proteins that tightly bind and hold onto them.

To make the connection even stronger, the researchers developed a nanoscale structure shaped a little like a tree, with each branch tipped with an antibody.

Therefore, as a cancer cell passes nearby, many individual branches can latch on, increasing the strength of the attachment.

This mechanism can capture an average of 200 circulating tumor cells from each milliliter of a patient's blood, many times the number of cells captured with previous technology.

It identified cancer cells in each of 24 patients undergoing treatment for head-and-neck, prostate, rectal or cervical cancer that enrolled in the study.

The study has also shown that, although the number of cells did not correlate with the stage, and thus severity, of the cancer, the reduction in cells is correlated with successful radiation therapy.