Purpose: Recently, an unprecedented 1-year 7% decrease in the overall incidence of invasive female breast cancer in the United States was reported. It has been suggested that the decrease resulted from the mass cessation of estrogen-progestin hormone therapy (EPHT) in 2002. We took advantage of California's unique population-based cancer surveillance resources to assess whether regional changes in breast cancer incidence observed between 2001 and 2004 correlated with regional changes in EPHT use between 2001 and 2003.
Methods: We obtained statewide cancer registry and California Health Interview Survey (CHIS) EPHT data for almost 3 million non-Hispanic white women age 45 to 74 years, residing in California's 58 counties. We examined trends in the age-adjusted incidence of invasive female breast cancer and compared these with trends in the use of EPHT, after grouping all California counties into three groups based on EPHT use in 2001. We also examined CHIS data on trends in screening mammography. Results In 2001, there were large regional differences in EPHT use and breast cancer incidence. From 2001 to 2004, incidence declined by 8.8% in the counties with the smallest EPHT reductions, by 13.9% in those with intermediate reductions, and by 22.6% in counties with the largest EPHT reductions. Between 2001 and 2003, CHIS data did not show any significant change in the proportion of women who reported having a mammogram in the previous 2 years.
Conclusion: These data support the hypothesis that changes in EPHT use in 2002 may be responsible for significant declines in breast cancer incidence between 2002 and 2003 and sustained through 2004.