TUESDAY, Nov. 30 (HealthDay News) -- Reduced use of hormone replacement therapy (HRT) is linked to declines in rates of invasive breast cancer and ductal carcinoma in situ, the most common form of noninvasive breast cancer, a new study reports.

The University of California, San Francisco, study included nearly 700,000 women, aged 40 to 79, who underwent more than 2 million screening mammography examinations between January 1997 and December 2008.

Women aged 50 to 69 had the highest level of hormone therapy use and also had the largest reduction in invasive breast cancer when they stopped hormone therapy -- from 40 cancers per 10,000 mammograms in 2002 to 31 cancers in 2005, and 35 cancers in 2006, the investigators found.

The study also found a sharp drop in rates of ductal carcinoma in situ in this age group after they stopped using hormone therapy.

A parallel decrease in these types of breast cancer also occurred among women older than 70. However, there was no change in breast cancer rates during the study period among women aged 40 to 49, who were less likely to have been on hormone therapy.

The researchers said their findings suggest that hormone therapy helped promote breast cancer tumor growth.

"We show that the incidence of breast cancer decreases if you take the hormones away. The fact that we're continuing to see a decrease in invasive [breast] cancer means that the effects of stopping the hormones may be long-lasting," senior author Dr. Karla Kerlikowske said in a university news release.

The study was released online in advance of publication in an upcoming print issue of the Journal of Clinical Oncology.

More information

The U.S. National Cancer Institute has more about hormone replacement therapy and cancer.