Children who have beaten cancer once are at increased risk of developing new tumors down the road, researchers say.

They found skin cancers, which are usually considered relatively harmless, appeared to be an early warning sign of more aggressive disease.

"It could be a marker for patients at significant risk," Dr. Gregory Armstrong of St. Jude Children's Research Hospital in Memphis, Tennessee, told Reuters Health.

"The take-home point is that the annual visit with your physician is very important, as is having your physician be aware of the guidelines," said Armstrong, whose findings appear in the Journal of Clinical Oncology.

According to the report, there were 328,000 survivors of childhood cancer in the U.S. in 2005 -- a number that keeps growing as treatment gets better.

The new study tracked more than 14,000 kids who had survived cancer for at least five years, for up to 38 years after their diagnosis.

One in twenty, or five percent, developed a new cancer during the study. Over the next 15 years following that diagnosis, the chance that a person who'd beaten cancer twice would get a third type of cancer was 12 percent. And the risk went up if the patients had received radiation as part of their treatment in childhood.

The researchers also found that survivors whose second tumor was non-melanoma skin cancer were twice as likely to get another aggressive cancer in the next 15 years compared to survivors with a different second tumor.

"We have been aware now for a couple a decades that children who beat their first cancer may be at risk of developing second cancers, largely as a result of the treatment they receive," said Armstrong.

He said the new results strengthen the case for starting cancer screening early in survivors, following guidelines developed by the Children's Oncology Group.

For instance, the group recommends starting mammograms at age 25 instead of age 40 or 50 if women have been treated for cancer with chest radiation as kids.

But it's not all bad news, said Armstrong.

"If you go back 50 years there were very few children surviving cancer," he explained. "Now we cure 80 percent of childhood cancers."