The incidence of advanced breast cancer among younger women, ages 25-39, may have increased slightly over the last three decades, according to a study released Tuesday.
But more research is needed to verify the finding, which was based on an analysis of statistics, the study's authors said. They do not know what may have caused the apparent increase.
Some outside experts questioned whether the increase was real and expressed concerns that the report would frighten women needlessly.
The study, published in the Journal of the American Medical Association, found that advanced cases climbed to 2.9 per 100,000 younger women in 2009, from 1.53 per 100,000 women in 1976 an increase of 1.37 cases per 100,000 women in 34 years. The totals were about 250 such cases per year in the mid-1970s and more than 800 per year in 2009.
Though small, the increase was statistically significant, and the researchers said it was worrisome because it involved cancer that had already spread to organs such as the liver or lungs by the time it was diagnosed, which greatly diminishes the odds of survival.
For now, the only advice the researchers can offer to young women is to see a doctor quickly if they notice lumps, pain or other changes in the breast and not to assume that they cannot have breast cancer because they are young and healthy, or have no family history of the disease.
"Breast cancer can and does occur in younger women," said Dr. Rebecca H. Johnson, the first author of the study and medical director of the adolescent and young adult oncology program at Seattle Children's Hospital.
But Johnson noted that there is no evidence that screening helps younger women who have an average risk for the disease and no symptoms. "We're certainly not advocating that young women get mammography at an earlier age than is generally specified," she said.
Expert groups differ about when screening should begin; some say at age 40, others 50.
Breast cancer is not common in younger women; only 1.8 percent of all cases are diagnosed in women from 20-34, and 10 percent in women from 35-44. However, when it does occur, the disease tends to be more deadly in younger women than in older ones.
Researchers are not sure why.
The researchers analyzed data from SEER, a program run by the National Cancer Institute to collect cancer statistics on 28 percent of the population of the United States. The study also used data from the past when SEER was smaller.
The study is based on information from 936,497 women who had breast cancer from 1976 to 2009. Of those, 53,502 were 25-39 years old, including 3,438 who had advanced breast cancer, also called metastatic or distant disease.
Younger women were the only ones in whom metastatic disease seemed to have increased, the researchers found.
Dr. Archie Bleyer, a clinical research professor in radiation medicine at the Knight Cancer Institute at the Oregon Health and Science University in Portland who helped write the study, said scientists needed to verify the increase in advanced breast cancer in young women in the United States and find out whether it is occurring in other developed Western countries.
"This is the first report of this kind," he said, adding that researchers had already asked colleagues in Canada to analyze data there.
"We need this to be sure ourselves about this potentially concerning, almost alarming trend," Bleyer said.
Johnson said her own experience led her to look into the statistics on the disease in young women. She had breast cancer when she was 27; she is now 44. Over the years, friends and colleagues often referred young women with the disease to her for advice.
Dr. Donald A. Berry, an expert on breast cancer data and a professor of biostatistics at the University of Texas' M.D. Anderson Cancer Center in Houston, said he was dubious about the finding, even though it was statistically significant, because the size of the apparent increase was so small 1.37 cases per 100,000 women, over the course of 30 years.
More screening and more precise tests to identify the stage of cancer at the time of diagnosis might account for the increase, he said.
"Not many women aged 25-39 get screened, but some do, but it only takes a few to account for a notable increase from 1 in 100,000," Berry said.