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A growing number of young American women are being diagnosed with advanced breast cancer, a new study has revealed.

Over the past three decades, the incidence of metastatic breast cancer in 25- to 39-year-old women has risen steadily from 1.53 per 100,000 in 1976 to 2.90 per 100,000 in 2009, according to the findings published Tuesday in the Journal of the American Medical Association.

"To our knowledge, this is the first time that this trend has been [observed] in American women," said the lead researcher, Dr. Rebecca Johnson, medical director of the adolescent and young-adult oncology program at Seattle Children's Hospital, which is affiliated with the University of Washington.

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"We think it is a real trend and, in fact, it seems to be accelerating," she added.

The results are based on an analysis of data from U.S. National Cancer Institute Surveillance, Epidemiology, and End Results (SEER) registries. In particular, the researchers looked at three separate cancer registries covering the periods from 1973-2009, 1992-2009 and 2000-2009.

Johnson said the trend can be seen in all ethnic groups, in both urban and rural areas as well as for both hormone-positive and hormone-negative types of tumours. (The growth of some tumours is fuelled by estrogen.)

Breast cancer is still a relatively uncommon disease among younger women. In terms of real numbers, the study suggests there were roughly 850 cases of metastatic cancer in the 25-to-39 age group in the United States in 2009, compared with 250 cases in 1975.

The research team described the change as being "a relatively small increase, but the trend shows no evidence for abatement." Younger women with breast cancer tend to experience more aggressive disease than older women and have lower survival rates.

Canadian cancer experts have not noticed a similar trend in this country. "This type of analysis hasn't been done for Canada," Prithwish De, an epidemiologist with the Canadian Cancer Society, said in a prepared statement. "There has been a recent commitment to better capture cancer stage-at-diagnosis information, so in the future this type of information should be more readily available for Canadians."

Meanwhile, the researchers are at a loss to explain why cancer incidence might be going up among younger women.

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"It is possible that it could be a change in lifestyle over the past several decades or potentially some sort of toxic exposure," Johnson speculated.

Previous studies have suggested that a combination of factors – including obesity, a high caloric intake and a sedentary lifestyle – can predispose women to develop breast cancer at a younger age.

But, she acknowledged, "It could be something quite different from the usual things we think of."

A study of the new chemicals that have entered widespread use in recent years could also help shed some light on the mystery.

"In my review of the literature – and environmental toxicology is not my specialty – I was surprised about how little data there seems to be about direct relationships between toxic exposures and specific types of human cancers," Johnson said.

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