Turning cancer cells into mini magnets by using nanoparticles could make biopsies so sensitive and efficient that there will be no need to repeat these invasive tests.

Biopsy results can be ambiguous: sometimes they can be negative simply because there are too few malignant cells in the sample to be detected - not because all trace of disease has gone. Now researchers from the University of New Mexico and the company Senior Scientific, both in Albuquerque, have come up with a solution that harnesses the power of magnetic attraction.

The idea is to use magnetic iron oxide nanoparticles encased in a biocompatible material. These in turn can be coated with antibodies that bind to chemicals found only in cancerous cells. When injected into the body, thousands of the particles stick to cancer cells, turning them into miniature magnets. The cells can then be drawn towards magnets encased in the tip of a biopsy needle (Physics in Medicine and Biology, vol 52, p 4009).

A mathematical model of the system confirmed that significant numbers of cancer cells, laden with nanoparticles, could be attracted to a needle within two or three minutes. In the lab, the researchers showed that a magnetised needle could attract leukaemia cells surrounded by nanoparticles and suspended in blood or other synthetic materials designed to mimic bodily fluids. Nanoparticles have been used before to destroy diseased cells but this is the first time they have actually retrieved cells.

The technique could benefit people with leukaemia, who must undergo regular bone marrow biopsies to check for signs of lingering disease. "Quite often, for example, in young children, doctors have to do several biopsies to get enough bone marrow," says Ed Flynn, president of Senior Scientific. It might also be possible to detect cells from breast, prostate, and ovarian cancers that have spread to other parts of the body in amounts too tiny to sample with an ordinary needle. The researchers are now seeking approval to test the method on human volunteers.

Bruce Morland, a paediatric oncologist at Birmingham Children's Hospital in the UK says that the idea would have to compete with techniques that have recently made it easier to detect tiny amounts of leukaemia cells in biopsies. "I am tempted to think that this technique will not be any more sensitive," he says. It might be used, however, to detect low levels of cancer that have spread "silently" into the bone marrow, he adds.