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 report Wednesday.

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

Reporting in Wednesday'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 Surveillance, Epidemiology and End Results database is a project of the National Cancer Institute.

Dr. Rebecca Johnson and colleagues revealed that 3 in every 100,000 young women nationwide develop 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.

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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.

There were 5,556 cases of advanced breast cancer in the SEER database during the study period. The years showing thelargest number of cases were 2000 to 2009.

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.

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."

Metastatic breast cancer is difficult to treat regardless of age but is particularly difficult in younger women, in whom the disease is more aggressive, doctors say.

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Dr. Brian O'Hea, director of the Carol M. Baldwin Breast Care Center at Stony Brook University, said knowing why cases of advanced disease are escalating is key.

He said the upward trend could be a result of better diagnostics, that more cases are detected because of better ways of finding cancer that has spread.

"Our methods of detecting metastatic disease have improved dramatically and substantially," O'Hea said Tuesday. "Fifteen, 20 years ago we didn't have PET scans; we did bone scans, and I don't think the technology of that era was catching everything."Citron, however, asked whether the instances of advanced disease were already advanced upon initial diagnosis or were recurrences after treatment for early breast cancer.

Relapses, he said, suggest the cancer became resistant to standard therapy. Citron said he sees about 100 Long Island women weekly in his practice who have breast cancer. Very few, he said, have advanced disease.

Breast cancer incidence and mortality statistics from the New York State Department of Health show that 61.8 women per 100,000 between the ages of 25 and 29 developed breast cancer -- regardless of stage -- between 2005 and 2009, the most recent years for complete statistics. Within that period, 184.4 women between the ages of 30 and 34 were diagnosed and 452 between the ages of 35 and 39.

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While mortality for the youngest group was 0.5 percent, women between the ages of 30 and 34 had a death rate of 2.4 percent. The last group's mortality was 7.5 percent.

"With better treatments emerging for breast cancer each year, it's disheartening to find that the rate at which young women are found to have metastatic disease on presentation is increasing," said Dr. Stephanie Bernik, chief of surgical oncology at Lenox Hill Hospital in Manhattan,

But Karen Miller, executive director of the Huntington Breast Cancer Action Coalition, said young women have been developing advanced disease for decades, probably even during the earliest period captured in the study.

"Doctors used to ignore young women, and many of them died because no one believed that a younger woman could get breast cancer," Miller said.