Survivors of childhood cancer face six times the usual risk of getting entirely new cancers in early adulthood - almost certainly because of the chemotherapy and radiation treatments that cured them, a large study found.

Doctors' ability to cure childhood malignancy has been one of the clearest successes of the war on cancer. About 1 in every 1,000 Americans in their 20s is a cancer survivor.

Several earlier reports have shown a surprisingly high cancer risk as young patients grow older, but the latest study, involving more than 13,000 survivors, gives the most comprehensive assessment yet of this unexpected downside of a medical victory.

When doctors began regularly curing childhood leukemia in the mid-1970s, they gave little thought to the possibility of distant ill effects. But because of the newly recognized risks, doctors now routinely try to use the least damaging treatment that will still cure the disease.

"It's clear that people treated for cancer in childhood are at increased risk of cancer later in life," said Dr. Joseph Neglia of the University of Minnesota. He presented the findings Tuesday at a meeting in New Orleans of the American Association for Cancer Research.

His research shows that while new cancer - especially breast cancer - occurs more often than expected in these patients, it still is rare. And the benefits of having cancer cured in childhood far outweigh any later risk.

Overall, the cancer survivors have a 3 percent risk of developing an entirely new cancer over the next 20 years. This is about six times greater than would be expected among people this age.

The researchers based their findings on a follow-up of 13,581 children and adolescents from 25 hospitals in the United States and Canada who had survived at least five years after treatment for leukemia and other forms of cancer. Their average age is now in the late 20s.

In all, 298 of the patients got new cancers. They were diagnosed an average of 12 years after their first malignancy. The most common new tumor was breast cancer, followed by thyroid and brain cancer.

In general, Neglia said, chemotherapy appears to increase the risk of new leukemia, while radiation boosts the risk of breast and other so-called solid tumors. These two main forms of treatment also probably work in combination to trigger cancer, probably by damaging patients' genes.

Among the study's findings:

Breast cancer was 16 times more common than expected and often occurred when women reached their late 20s and 30s. The researchers recommended that girls who got radiation to their chests have a mammogram by age 25.

Bone cancer was 19 times more common than usual and thyroid 11 times more common among the cancer survivors.

The highest extra cancer risk was seen in children who were treated for Hodgkin's disease. They had an almost 8 percent chance of new cancer during 20 years of follow-up. The risk was lowest among longtime survivors of non-Hodgkin's lymphoma.

The same underlying genetic defect that triggered the patient's original cancer might have caused some of the new tumors. But the researchers believe chemotherapy and radiation were largely to blame.

Doctors say they try to minimize exposure to toxic treatments as much as possible, especially radiation. This has largely been abandoned as a treatment for Hodgkin's disease and some leukemias. Now, about one-quarter of young cancer patients get radiation, compared with half 15 years ago.

"We really have to worry about the children we are curing," said Dr. Barton Kamen of New Jersey's Robert Wood Johnson Medical School.

Still, he said, doctors must treat aggressively enough to cure patients on the first attempt, because the disease rarely can be eliminated once it returns in these patients.

Between 8,000 and 10,000 new cases of childhood cancer are diagnosed in the United States each year. About 70 percent are cured.

Dianne Traynor of the Pediatric Brain Tumor Foundation noted that childhood cancer survivors may face a variety of other health problems, including learning problems, epilepsy and permanent hair loss.

The latest findings, she said, "will certainly spur new research efforts. They show the important of new treatments that are less invasive."