Children's kidney and thyroid cancers have increased in recent years, and though the diseases are rare, experts wonder if the rising rates could be related to obesity.

The rate for all childhood cancers combined, 171 cases per million children, remained stable from 2000 to 2009 although slight increases were seen in blacks and adolescents, according to a report from researchers at the federal Centers for Disease Control and Prevention.

During the years studied, there were about 120,000 new cancer diagnoses among infants and children aged 1 through 19.

Mostly stable rates of leukemia and brain tumors, among the more common types of childhood cancer, contributed to the overall trend.

For thyroid cancer, the diagnosis total was almost 5,000 cases, and the rate climbed from nearly six cases per million children to eight per million children in 2009.

For kidney cancer, the total was 426 and the rate climbed from 0.5 cases per million to 0.7 cases per million.

Studies showing increases in both kinds of cancer in adults have suggested causes might include rising obesity rates. Childhood obesity also has increased in recent years and whether that has contributed to the rising cancer rates needs to be determined, the authors of the new study said.

Study co-author Dr. David Siegel emphasized that the increases were relatively small and that children's overall risks for developing cancer are low. Siegel worked on the study while at the CDC but has since moved to Emory University and Children's Healthcare of Atlanta.

He said the researchers used data through 2009 because more recent data contained inconsistencies.

The report was published Monday in Pediatrics.