A collaborative effort involving U.S. scientists and private companies are looking into a test that could find even one stray cancer cell among the billions of cells that circulate in the human bloodstream.

The trust is that one day such a test, given soon after a treatment is started, could indicate whether the therapy is working or not. It might even indicate beforehand which treatment would be most effective.

The test relies on circulating tumor cells (CTCs) cancer cells that have detached from the most important tumor and are traveling to other parts of the body.

In 2007, researchers at Massachusetts General Hospital, developed a "microfluidic chip," called CellSearch, which could calculate the number of stray cancer cells, but that test didn't allow scientists to trap whole cells and analyze them.

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