Aug. 25 (HealthDay News) -- The first accounting of women with breast cancer in situ in the United States finds that in 2005 there were 610,171 survivors, but that by 2016 that number is expected to increase to more than 1 million.

Breast cancer in situ now accounts for 20 percent of newly diagnosed breast cancers. It is the early stage of the disease, when it is still confined to the layer of cells in the ducts or lobules of the breasts.

University of Wisconsin-Madison researchers Brian L. Sprague and Amy Trentham-Dietz noted that while there were 2.5 million breast cancer survivors in the United States in 2005, the number of breast cancer in situ survivors was unknown. Their research is reported in the Aug. 26 issue of the Journal of the American Medical Association.

"A lot, but not all, of the increase in in situ breast cancers is a consequence of screening for breast cancer," explained Barbara A. Brenner, executive director of the advocacy group Breast Cancer Action. "This is leading to a lot of over-diagnosis, and hence, over-treatment."

Using data from the Surveillance, Epidemiology and End Results registries, the Wisconsin researchers were able to estimate the number of surviving women with breast cancer in situ and predict the trend over the next decade.

They found that in 2005, 610,171 of the women diagnosed with breast cancer in situ in the previous 30 years were alive.

There are two types of breast cancer in situ: ductal and lobular. These women are four times more likely to develop invasive breast cancer compared with the general population, the researchers noted.

Since there is no way to predict which women will develop invasive breast cancer, they often have the same treatment as women with localized invasive cancer with similar reductions in quality of life.

"There is currently no way to tell women with ductal breast cancer in situ or lobular breast cancer in situ whether their condition, untreated, will develop into invasive disease," Brennen said. "So, knowing you have either condition just puts you in a world of 'What to do now.'"

Women with ductal breast cancer in situ were more than five times more likely to be survivors compared with women diagnosed with lobular breast cancer in situ, the Wisconsin researchers found.

In addition, more white women survived than black women and women from other ethnic groups, the researchers found.

Sprague and Trentham-Dietz also found that survival has increased over time. For example, 21,654 women diagnosed in 1975 were still alive in 2005, while that number quadrupled for women diagnosed in 1985 and doubled again for women diagnosed in 1995.

"Assuming constant incidence and survival rates, the estimated prevalence of breast cancer in situ will exceed 1 million cases by 2016," the authors wrote.

Improved survival may be due in part to better screening and earlier diagnosis, the researchers noted. Before 1990, breast cancer in situ made up only 4 percent of all breast cancer diagnoses.

"If there is an underlying increase in in situ breast cancers that is not the result of screening -- and there seems to be -- we should be looking for the causes -- very likely environmental -- of that increase," Brenner said.

The researchers agreed that there is still a lot more to learn about in situ breast cancer.

"Current epidemiologic evidence regarding predictors of subsequent invasive breast cancer after breast cancer in situ is limited," Sprague and Trentham-Dietz wrote. "Guidelines are necessary to help the increasing number of breast cancer in situ survivors choose the best treatment and lifestyle strategies while still maintaining high quality of life."

SOURCES: Barbara A. Brenner, executive director, Breast Cancer Action, San Francisco; Aug. 26, 2009, Journal of the American Medical Association