Previously, scientists had thought viable eggs could only be obtained from girls who had undergone puberty.
"We didn't expect young girls to have eggs that could withstand the process of maturation," which involves adding hormones, said Dr. Ariel Revel, who led the research at the Hadassah Hospital in Israel.
The research will be presented Tuesday at the annual meeting of the European Society of Human Reproduction and Embryology in Lyon, France.
In related work, Canadian doctors announced Monday the first birth of a baby from eggs matured in a laboratory, frozen, thawed and then fertilized — a key development that holds promise for infertile women.
The year-old baby girl was born to a woman in Canada, doctors told the Lyon conference. Three other women are pregnant from eggs that had been matured in a lab, frozen, thawed and then implanted, they said.
The women in the study are infertile with an average age of about 30. None has a history of cancer. Until now, doctors did not know whether eggs matured in a lab could withstand the fertilization process, adding that the research is still in early stages.
In the study involving young girls with cancer, Revel surgically extracted the eggs and then artificially matured them in a laboratory, with the idea of re-implanting them one day should the patient wish to have children.
To obtain the eggs, Revel and his colleagues performed surgery on 18 patients ages 5 to 20. Of 167 eggs, 41 were successfully matured, including some from prepubescent donors. They were then indistinguishable from those of older women, Revel said.
"Any advance that enables young women to have children one day after having cancer is positive," said Simon Davies, head of Teenage Cancer Trust, a charity based in Britain. Davies was not linked to the research.
But as the extraction of eggs is an invasive operation, Davies said more information was needed about potential risks to young women fighting cancer. There might also be ethical concerns, as the decision to remove eggs from very young girls would likely be made by the parents, not the patient.
Experts think cancer treatments can affect female fertility. Chemotherapy usually affects all body cells, attacking not only the cancer, but other areas including the ovaries — for which it is often deadly.
Unlike men, who produce sperm throughout their lifetime, women only have a set number of eggs from their birth; that number decreases as they age. Young girls who undergo aggressive chemotherapy treatments often experience a sharp drop in the number of their eggs, and some become completely infertile.
The cure rate for childhood cancer can be as high as 90 percent, and doctors are investigating options for preserving patients' fertility. Another experimental method involves removing a thin layer of ovarian tissue for re-implantation later, but trials so far have resulted in only a handful of pregnancies worldwide.
"The research by Dr. Revel is an important option for prepubescent girls who may otherwise lose the ability to have children," said Dr. Hananel Holzer of the obstetrics and gynecology department at McGill University in Montreal.
Holzer, who was not connected to Revel's study, said immature eggs from adult women have previously been matured in the laboratory, but until now, no one had ever tried it with eggs from young girls.
The real test will come when the girls might be ready to have children. "We will only know the final chapter of this story in about 10 years, when we hope to close the circle of this research," Revel said.
None of the eggs has yet been thawed, and experts are unsure if the artificial insemination process could result in other problems such as chromosomal abnormalities. Additional surveillance, such as amniocentesis screenings to check the baby's development, would probably be necessary.