The percentage of young women  in the U.S. who have advanced breast cancer has increased slightly in recent decades, according to a new study.

Between 1976 and 2009, the rate of metastatic breast cancer (cancer that has spread to distant organs such as bones or the brain) among women ages 25 to 39 increased by about 2 percent each year, the study found.

While this increase is small — it translates to about 1.4 extra cases per 100,000 people over a 34-year period — it shows no signs of tapering off, the researchers said.

Because this is the first time researchers have seen an increase in advanced breast cancer in the U.S., the findings will need to be confirmed by future studies. But if real, the results are particularly alarming because young women with advanced breast cancer have a low survival rate (about 31 percent of them survive five years). What's more, young women are not advised to receive breast cancer screening unless they have a family history of the disease, the researchers said.

The reason for the increase is also not clear, and will need to be investigated further,  said study researcher Dr. Rebecca Johnson of Seattle Children's Hospital.

Breast cancer in young women is less common than it is in older women: about 1 in 227 American women will develop the disease before age 30; 1 in 42 will be diagnosed with it by age 50; and 1 in 28 will develop it by age 60, according to the National Cancer Institute.

In the new study, Johnson and colleagues analyzed information on breast cancer incidence using databases from the National Cancer Institute.

The researchers found that the rate of metastatic breast cancer among women ages 25 to 39 increased from 1.53 cases per 100,000 people in 1976 to 2.90 cases per 100,000 people in 2009. The biggest increase was seen during the most recent decade. Among all young women with breast cancer, the proportion diagnosed with metastatic breast cancer increased from 4.4 percent in the 1970s to 5.5 percent in the 1990s and 7.2 percent in the 2000s.

By contrast, there was no increase in early-stage breast cancer in young women, or breast cancer of any stage in older women, the researchers said.

A possible reason for the new increase could be that what doctors call metastatic breast cancer is being diagnosed at a different stage now than it used to be. But if that were the case, we would expect to see decreases in the number of women diagnosed with earlier stages of breast cancer, which has not happened, Johnson said.

The researchers examined existing studies to see if they could find an environmental factor that changed during the same period when the increase occurred, but couldn't find one, Johnson said. The increase in obesity in recent years has been a significant change, but earlier studies actually found a decreased risk of breast cancer in obese young women. If the increase is real, it's likely that more than one factor is responsible, Johnson said.

A 2007 study in Switzerland also detected an increase in breast cancer among young women. Data supporting an increase in breast cancer in young women may have to come from other countries, as the largest database documenting cancer incidence in the United States only goes back to the 1970s, Johnson said.

Pass it on: The rate of advanced breast cancer among young American women has increased slightly in recent years.

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