Chicago ? Surprising research suggests that childhood cancer is most common in the Northeast, results that even caught experts off guard. But some specialists say it could just reflect differences in reporting.

The large government study is the first to find notable regional differences in pediatric cancer. The study from the Centers for Disease Control and Prevention is based on data representing 90 percent of the U.S. population. It found that cancer affects about 166 out of every million children, a number that shows just how rare childhood cancers are.

The highest rate was in the Northeast with 179 cases per million children, while the lowest was among children in the South with 159 cases per million. Some experts suggested that could mean cases were under-reported in the South and over-reported elsewhere.

The rates for the Midwest and West were nearly identical, at 166 cases per million and 165 per million, respectively.

The cancer incidence in boys was 174 cases per million, compared with 157 cases per million in girls. In white children, the rate was 173 per million, versus 164 per million in Hispanics and 118 per million in blacks. Teenagers had higher rates than younger kids.

A total of 36,446 cases were identified in the study, which analyzed 2001-03 data from state and federal registries. The research appears in the June edition of Pediatrics, released today.

“It’s very powerful that this study includes so much of the U.S. population so it gives us a good picture of where we are with the incidence of these childhood cancers,” said Elizabeth Ward, the American Cancer Society’s surveillance director.

Experts said the regional differences, though small, are intriguing, but that reasons for them are uncertain.

Environmental factors might play a role, including exposure to radiation, said lead author Dr. Jun Li of the CDC. Radiation has been linked with the most common types of childhood cancer – leukemia, lymphoma and brain cancers.

Dr. Lindsay Frazier, a cancer specialist at Children’s Hospital Boston and Dana Farber Cancer Institute, said pollution and housing stock that’s older than anywhere else in the nation might help explain the Northeast’s higher rates.

But also, there could be better access to cancer centers in the Northeast, which would result in more diagnoses, she said. That could also explain why other research has shown that children’s death rates from cancer are also lowest in the Northeast.