A family desperate to save a child from a lethal brain disease sought highly experimental injections of fetal stem cells, a procedure that triggered tumours in the boy's brain and spinal cord, Israeli scientists reported Tuesday.

Scientists are furiously trying to harness different types of stem cells — the early-stage building blocks that differentiate into other cells in the body — to regrow damaged tissues and thus treat devastating diseases. But for all the promise, researchers have long warned that they must learn to control newly injected stem cells so they don't grow where they shouldn't, and small studies in people are only just beginning.

Tuesday's case report in the journal PLoS Medicine is the first documented case of a human brain tumour — albeit a benign, slow-growing one — after fetal stem cell therapy. The journal is published by the Public Library of Science.

"Patients, please beware," Dr. John Gearhart, a stem cell scientist at the University of Pennsylvania who wasn't involved in the Israeli boy's care, told the Associated Press.

"Cells are not drugs. They can misbehave in so many different ways, it just is going to take a good deal of time" to prove how best to pursue the potential therapy, Gearhart said.

Boy suffering from rare genetic disease

The unidentified Israeli boy has a rare, fatal genetic disease called ataxia telangiectasia, or AT. Degeneration of a certain brain region gradually robs these children of movement. The disorder also results in a faulty immune system, which leads to frequent infections and cancers. Most die in their teens or early 20s.

Israeli doctors pieced together the child's history. When he was nine, the family travelled to Russia, to a Moscow clinic that provided injections of neural stem cells from fetuses — immature cells destined to grow into a main type of brain cells. The cells were injected into his brain and spinal cord twice more, at ages 10 and12.

Back home in Israel at age 13, the boy's AT was severe enough to require that he use a wheelchair when he also began complaining of headaches. Tests at Sheba Medical Center in Tel Aviv uncovered a growth pushing on his brain stem and a second on his spinal cord.

Surgeons removed the spinal cord mass when the boy was 14, in 2006.

Stem cells mixed with growth-spurring compounds

But scientists weren't initially sure if the boy was prone to tumours anyway or if fetal stem cells were to blame. A Tel Aviv University team extensively tested the tumour tissue and concluded the fetal cells were at the root of the problem.

Among other evidence, some of the cells were female and had two normal copies of the gene that causes AT, although the boy's underlying poor immune function could have allowed the growths to take hold.

Using stem cells from multiple fetuses that also were mixed with growth-spurring compounds "may have created a high-risk situation where abnormal growth of more than one cell occurred," wrote lead researcher Dr. Ninette Amariglio of Sheba Medical. She urged better research to "maximize the potential benefits of regenerative medicine while minimizing the risks."

While the study authors spoke of the need for caution in stem cell therapy, they concluded that their findings "do not imply that the research in stem cell therapeutics should be abandoned."

AT wasn't conducive to stem cell therapy in the first place, said stem cell specialist Dr. Marius Wernig of Stanford University, who said it's unclear exactly what was implanted.

"Stem cell transplantations have a humongous potential," Wernig said. But, "if people rush out there without really knowing what they're doing ... that really backfires and can bring this whole field to a halt."