June 26 (HealthDay News) -- Cases of invasive breast cancer in the United States have declined overall, but the decrease is significantly less marked in poor women who live in rural areas, possibly due to differences in the use of hormone therapy (HT), a new study suggests.

Researchers at the Northern California Cancer Center studied national breast cancer incidence data for the years 1997 to 2004 to compare rates in rural and urban areas and poor and rich areas of the country.

"Between 2001 and 2004, incidence rates of invasive breast cancer declined more than 8 percent in the United States," study leader Christina Clarke said in a news release. "One possible explanation for this is widespread discontinuation of and/or failure to initiate HT. Because this cessation of HT use was more pronounced in rich/urban areas, we wanted to see if there was a corresponding difference in breast cancer incidence between these areas and poor/rural parts of the country."

She and her colleagues found that overall invasive breast cancer incidence declined 13.2 percent, with notable differences between urban (-13.8 percent) and rural (-7.5 percent) women, and those in counties with high (-9.6 percent), middle (-13.8 percent) and low (-13 percent) poverty rates.

The researchers noted that breast cancer incidence rates in rural counties peaked in 1999 and then declined steadily, while rates in urban counties fell dramatically after 2002. This difference may be because of variations in levels of exposure to news linking hormone therapy with breast cancer. The fact that women in urban areas had more exposure to media reports about the HT/breast cancer relationship likely contributed to a larger decrease in hormone therapy among those women.

"Understanding what specific populations were involved in breast cancer declines helps us to better plan prevention efforts for the future, especially with the aging of the baby boomer population into prime breast cancer age," Clarke said.

The study appears in the journal BMC Medicine.

SOURCE: BioMed Central, news release, June 25, 2009