The rate of advanced breast cancer for U.S. women 25 to 39 years old nearly doubled from 1976 to 2009, a difference too great to be a matter of chance, a study finds.

Rebecca Johnson was 27 years old and had just graduated from medical school when she got the diagnosis: breast cancer. She thought she was a rare case, but then a few of her friends got it too. So did some friends of friends.

Was it all just a coincidence, or was breast cancer becoming more common in younger women?

“I really wondered,” said Johnson, now 44 and the director of the Adolescent and Young Adult Oncology program at Seattle Children’s Hospital. So she examined decades’ worth of data from the National Cancer Institute and made a disturbing find: Cases of younger women with advanced breast cancer have increased about 2% each year since the mid-1970s and show no signs of abating.

The results, published in Wednesday’s edition of the Journal of the American Medical Assn., confirmed the suspicions of many oncologists who had noticed an uptick in patients younger than 40 with cancer that had spread to the bones, brain or lungs.

In 1976, 1.53 out of every 100,000 American women 25 to 39 years old was diagnosed with advanced breast cancer, the study found. By 2009, the rate had almost doubled to 2.9 per 100,000 women in that age group - a difference too large to be a chance result.

“Most studies have failed to show an absolute increase,” said Dr. Benjamin Paz, a City of Hope Cancer Center surgeon who was not involved in the study. “Now, looking at a longer period of time, this study shows there’s clearly been an increase. It’s the first to do so.”

The trend, which has yet to be explained, has raised real concerns about future efforts to treat the disease. Survival rates for young women with metastatic breast cancer are much lower than they are for older women, because the cancer tends to behave more aggressively in the young.

The data from the U.S. Surveillance, Epidemiology and End Results, or SEER, database detected a significant increase among black and white women in both urban and non-urban areas, suggesting that the root cause or causes were widespread.

“An increasing number of young women in the United States will present with metastatic breast cancer in an age group that already has the worst prognosis, no recommended routine screening practice, the least health insurance, and the most potential years of life,” Johnson and her coauthors wrote.

To be sure, it remains uncommon for a young woman to be diagnosed with breast cancer. About 7% of all breast cancers diagnosed in the United States involve women younger than 40, and on average, 1 in 173 women in this age group risks developing some type of breast cancer.

Johnson and her coauthors said they hoped that other Western nations would corroborate their findings using their own data. If a trend is established, research should investigate the reason for the increase, they added.

They hypothesized that the trend was due to a variety of lifestyle changes that have occurred during the study period. Diet, exercise, obesity, earlier onset of menstruation, use of birth control, delayed pregnancy and other factors all might play a role.

A few smaller studies have examined risk factors such as obesity, sedentary lifestyle and high caloric intake and concluded that when combined, they do predispose to young adult breast cancer.

However, it is still unknown exactly why cancers can behave so much more aggressively in younger patients, and why estrogen - or the blocking of it - has a very different effect on cancer cells in younger and older women.

“There’s something different about breast cancers in young adults than in older people,” Johnson said. “Researchers that are focusing on cancer in young adults are trying to tease out what those biological differences are.”

In the meantime, she said she hoped the study would alert young women to the risks of breast cancer.

“There’s no evidence that 29-year-olds should go out and get mammograms or anything like that,” Johnson said. “But if there’s a take-home message, I would say that it would be awareness of the fact that breast cancer can happen even in young women and that it’s important for both young women and their doctors to be aware of this.”

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