Low doses of radiation therapy to the head, for brain tumors or some other cancers, might make it slightly harder for girls to have babies when they get older, a new study shows.

Doctors already knew that higher doses of radiation for these conditions can interfere with a woman's fertility, because it can destroy cells in the brain that control how her ovaries produce eggs, said Dr. Daniel Green, a cancer doctor at the St. Jude Children's Research Hospital in Memphis, Tennessee.

"It was not known that more modest doses can possibly have an effect on fertility," Green, who was a co-author on the study, told Reuters Health.

Most of the girls in the study, about 6 out of 10, had either leukemia or a brain tumor. Others had cancers of the lymph nodes, bone, or muscles.

None of the girls had radiation of their ovaries, which might have affected their fertility.

Instead, Green and his colleagues wanted to know what happens to the fertility of girls who have radiation that reaches two glands next to the brain, the pituitary and the hypothalamus.

Leukemia -- cancer of the white blood cells -- is sometimes treated this way if cancer cells show up in the fluid that bathes the spinal cord and the brain.

"For brain tumors, radiation has always been part of the treatment," although doctors are using more chemotherapy now than they have in the past, said Dr. Jason Fangusaro, a pediatrician at Children's Memorial Hospital and Northwestern University Feinberg School of Medicine in Chicago, Illinois.

This type of radiation isn't used for leukemia patients as often now as it once was, Fangusaro, who wasn't involved in the study, told Reuters Health.

But there are "many women who were treated for this 10 or 15 years ago" who could be at risk for fertility problems, Green said, and doctors don't know exactly what their numbers are.

In their study, Green and colleagues sent questionnaires to about 3,600 women who had cancer as girls, between 1970 and 1986, and also to about 2,100 of their sisters, asking about any pregnancies the women might have had.

About 3 of every 10 former cancer patients had been pregnant at least once, compared to about 5 out of 10 of their sisters, the researchers report in the medical journal Fertility and Sterility. This small difference, they say, could have been due simply to chance.

Green's group did not analyze how many times women tried to become pregnant.

But the actual pregnancy rate was significantly lower for some of the women - and not just those who received the highest doses of radiation to those two glands near the brain.

The highest doses involved more than 27 units of radiation, known as Grays. But women exposed to at least 22 units of radiation also had significantly lower odds of pregnancy; they had about a third fewer pregnancies compared to survivors who hadn't had been treated with radiation at all.

A dose of 22 Grays is more than 200,000 times higher than the amount of radiation in an average chest X-ray.

Leukemia is the most common type of cancer in kids, and about 3,300 new cases were expected in 2010, according to the Leukemia and Lymphoma Society.

And kids develop about 3000 to 4000 new brain tumors a year, Fangusaro said.

When cancer survivors do become pregnant, "it's considered a high-risk pregnancy, and they need to be seeing a specialist who knows about helping the woman ovulate," and maintaining the pregnancy with the right hormone therapy, said pediatrician Dr. Susan R. Rose. Rose, from Cincinnati Children's Hospital in Ohio, was not involved in the study.

Research advances have improved the odds of survival after childhood cancers. So if a girl was diagnosed with cancer in the past, it is likely she's still alive and may be facing fertility problems, Rose told Reuters Health.

"It's also likely that if she didn't get (the radiation) treatment, she might have not survived," she said. "It's part of a lifesaving therapy."