A woman in the US has developed a tumour-like growth eight years after a stem cell treatment to cure her paralysis failed. There have been a handful of cases of stem cell treatments causing growths but this appears to be the first in which the treatment was given at a Western hospital as part of an approved clinical trial.

At a hospital in Portugal, the unnamed woman, a US citizen, had tissue containing olfactory stem cells taken from her nose and implanted in her spine. The hope was that these cells would develop into neural cells and help repair the nerve damage to the woman's spine. The treatment did not work far from it. Last year the woman, then 28, underwent surgery because of worsening pain at the implant site.

The surgeons removed a 3-centimetre-long growth, which was found to be mainly nasal tissue, as well as bits of bone and tiny nerve branches that had not connected with the spinal nerves.

The growth wasn't cancerous, but it was secreting a "thick copious mucus-like material", which is probably why it was pressing painfully on her spine, says Brian Dlouhy at the University of Iowa Hospitals and Clinics in Iowa City, the neurosurgeon who removed the growth. The results of the surgery have now been published.

"It is sobering," says George Daley, a stem cell researcher at Harvard Medical School who has helped write guidelines for people considering stem cell treatments. "It speaks directly to how primitive our state of knowledge is about how cells integrate and divide and expand. "

The case shows that even when carried out at mainstream hospitals, experimental stem cell therapies can have unpredictable consequences, says Alexey Bersenev, a stem cell research analyst who blogs at Cell Trials. "We have to realise complications can also happen in a clinical trial," he says.

Stem cells have the prized ability to divide and replenish themselves, as well as turn into different types of tissues. There are several different stem cells, including ones obtained from an early embryo, aborted fetuses, and umbilical cord blood. There are many sources within adult tissues, too, including bone marrow.

While often hailed as the future of medicine, stem cells' ability to proliferate carries an inherent danger and the fear has always been that when implanted into a person they could turn cancerous.

Still, a few stem cell therapies have now been approved, such as a treatment available in India that takes stem cells from the patient's eye in order to regrow the surface of their cornea, and a US product based on other people's bone stem cells.

Many groups around the world are investigating a wide range of other applications, including treating heart attacks, blindness, Parkinson's disease and cancer. Research groups at universities and hospitals need to meet strict safety guidelines for clinical trials but some small private clinics are offering therapies to people without research or marketing approval. There is a growing number of lawsuits against such clinics and a few cases have been reported of tumours or excessive tissue growth (see "Ongoing stem cell trials" below).

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Stem cell treatment causes nasal growth in woman's back