MONDAY, Jan. 3 (HealthDay News) -- A collaborative effort involving U.S. scientists and private companies is looking into a test that could find even one stray cancer cell among the millions of cells in a given blood sample.

The hope 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, and others like it under development, relies on circulating tumor cells (CTCs) -- cancer cells that have detached from the main tumor and are traveling to other parts of the body.

Other versions of tests aimed at spotting CTCs are already in the works. For example, a few years ago, researchers at Veridex LLC, a Johnson & Johnson company, developed a technology called CellSearch, which counts the number of stray cancer cells in blood. However, that test didn't allow scientists to trap whole cells and analyze them.

And at Massachusetts General Hospital, researchers have developed a type of microchip with a surface covered with antibodies that are designed to stick to tumor cells. As blood passes over the chip, tumor cells separate from the pack and adhere to the surface.

Then, on Monday, Mass General announced an agreement with Veridex to develop and study a wholly new type of CTC blood test that involves a different, and as yet undisclosed, technology. Media reports on Monday had erroneously stated that the test involved microchip technology, hospital officials said.

Scientists are wagering that these types of test, if successful, might also detect cancer early in its course, predict the odds for a recurrence, and assess a patient's general prognosis.

"There has been speculation that these [stray] cells are the ones that are responsible for the spreading of the disease," said one expert, Dr. Massimo Cristofanilli, professor and chairman of medical oncology at Fox Chase Cancer Center in Philadelphia. "Simple enumeration tells us that this patient has a worse prognosis . . . Now the question is, what other information we can gather, if we are able to capture these cells? For example, could we do gene analysis profiling and can we get information for the best treatment?"

As it stands today, biopsy -- an invasive and sometimes even hazardous procedure -- is one of the few ways doctors can get key information about a cancer's size and characteristics.

"Many people consider [the new blood test to be] a 'liquid biopsy,' so that eventually we can access cancer cells that are representative of the tumor without performing an invasive biopsy," said Cristofanilli, who is not involved in developing the test.

Experts stressed that the new type of test, if it ever arises, may still be years away, and researchers still aren't sure what these circulating tumor cells (CTCs) actually mean.

"They may be able to detect small amounts of cancer cells but we don't know the significance of that. We may be detecting things that don't have clinical significance," explained Dr. Jay Brooks, chairman of hematology/oncology at Ochsner Health System in Baton Rouge, La.

And as Cristofanilli pointed out, these plans so far are "only for research. The test is not available for clinical use." Later this year, in fact, four major cancer centers -- Mass General, Memorial Sloan-Kettering Cancer Center in New York City, the University of Texas' M.D. Anderson Cancer Center in Houston, and the Dana-Farber Cancer Institute in Boston -- will begin studies using the latest microchip-based CTC test developed by Mass General.

Any such test would need to be developed "along with the process of new drug development and new targeted therapies so we can better use the information with a clinical purpose," Cristofanilli added.

More information

There's more on cancer's spread at the U.S. National Cancer Institute.