NEW YORK - Advanced breast cancer in young women has been rising for three decades, a trend occurring among all ethnicities that defies a marked decline in late-stage disease in older women, researchers reported Wednesday.

Although the upswing is slight, experts Tuesday called it statistically significant - and worrisome.

Reporting in today's Journal of the American Medical Association, investigators from the Adolescent and Young Adult Oncology program at Seattle Children's Hospital in Washington pinpointed the trend in women between the ages of 25 and 39.

Numbers illustrating it were drawn from the vast SEER database, a registry of cancers of all types that have occurred in the United States since 1973. The database is a project of the National Cancer Institute.

Dr. Rebecca Johnson and colleagues revealed that 3 in every 100,000 young women nationwide develops advanced breast cancer. It can spread to the bones, liver, lungs or brain.

Johnson found that advanced cases increased to nearly 3 women per 100,000 in the population in 2009, up from 1.53 per 100,000 in 1976, an increase of about 1.37 cases per 100,000 over the 34-year period.

Looking at those numbers another way, doctors saw about 250 advanced breast cancer cases in 1976 compared with more than 800 by 2009. All told, they studied more than 930,000 cases in that period.

The study's authors could not explain the increase.

Dr. Mark Citron, who led the Long Island arm of a large nationwide study, which culminated in the approval of Kadcycla, a new therapy for advanced breast cancer last week, said the findings are in sharp contrast to all other breast cancer trends.

"This is contrary to what has been seen generally," said Citron, director of cancer services for ProHEALTH Care Associates in Lake Success, N.Y.

Citron, a former head of medical oncology at Long Island Jewish Medical Center, said further research into the findings is vital because advanced breast cancer is the worst possible diagnosis.

"It's not curable," Citron said. "Patients may go into long-term remissions, but once it has advanced, it's an incurable disease."

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