The effort to discover cancer in its early stages, when it is more easily treated, could get a boost from a highly sensitive new blood test.

Tumors begin sending out cancer cells early in their development, and the new test is much more sensitive to these cells than current methods, the researchers reported in Proceedings of the National Academy of Sciences.Dr. Jonathan Uhr of the University of Texas Southwestern Medical Center at Dallas said he cannot yet say how much earlier a tumor might be discovered.

But Dr. Emilian Racila, one of his fellow researchers, noted that some breast cancers can grow for years before becoming large enough to be detected by mammograms.

The new test uses magnets to concentrate the cancer cells from blood and lasers to observe them. It has been used on patients with breast and prostate cancer, and tests on other tumors are planned.

Dr. Ronald Ghossein of Memorial Sloan-Kettering Cancer Center in New York, who was not connected to the study, called the findings "promising."

And Dr. Carleton Stewart of Roswell Park Cancer Institute in Buffalo, N.Y., who also was not among the researchers who wrote the new report, termed the results "extremely positive."

Stewart said similar research is under way at his hospital with results much like those reported by Uhr and his co-workers.

"It is not only a means of early detection and monitoring, but it can have a significant impact on treatment plans for the patient. If one knows that the patient has circulating (cancer) cells, they may require a different kind of treatment than one who doesn't," he explained.

Uhr said the researchers want to see if the test, besides offering the hope of earlier detection for breast and prostate cancer, works for cancers of the colon and intestinal tract, which are harder to detect with traditional methods.

The test locates epithelial cells, a type of cell that forms the skin, glands and other tissues but is rarely present in the bloodstream.

Researchers hope the test will be useful in following the progress of cancer treatment by determining if the number of epithelial cells in the blood declines. And any sudden increase after treatment ends could warn of impending relapse.

Results have been encouraging, but Uhr cautioned that "a year or two of further study" is needed to determine if the test will help physicians in dealing with individual patients.

While doctors have long known that tumors shed cells into the bloodstream, it is only after the cancer has reached a certain size that enough cells are present to be detected.

With the new test, tiny iron particles coated with an enzyme that attaches itself to cancer cells are placed in the blood sample. Magnets are used to concentrate the cancer cells, which can then be studied.

Breast and prostate cancers tend to be slow-growing, but shed these epithelial cells from their early development, Racila explained. While a breast tumor might contain 100 million cells before it could be found in a mammogram, a tumor of 1 million cells could shed enough epithelial cells into the blood to be detected, Racila said.

The researchers found that blood samples from 13 healthy people averaged just 1.5 epithelial cells per sample. By comparison 14 patients with breast cancer that had not spread averaged 15.9 epithelial cells; five with cancer that had spread locally averaged 47.4 cells and 11 patients whose cancer had spread to other parts of the body averaged 122 cells.