According to results recently presented at the 2006 annual San Antonio Breast Cancer Symposium, researchers speculate that a sudden decrease in the incidence of breast cancer in 2003 may be due to a reduction in the use of hormone replacement therapy (HRT) among postmenopausal women. 

In 2002, results from the Women’s Health Initiative (WHI) were initially announced, indicating that HRT increases the risk of breast cancer. It is speculated that rates of HRT use among postmenopausal women dropped by at least half to nearly 70% following the release of the WHI results.

Researchers from the National Cancer Institute and Harbor-University of California, Los Angeles Medical Center recently conducted a study that evaluated the incidence of breast cancer by using the Surveillance Epidemiology and End Results (SEER) database in 9 regions in the United States. According to this data, the incidence of breast cancer had been increasing 1.7% each year from 1990-1998. After 1998, breast cancer rates began to decline by 1% each year. However, in 2003, there was a 7% decline in breast cancer incidence.

Overall in 2003, there was an 8% decline in the rates of estrogen-receptor (ER) positive breast cancer, and a 4% decline in the incidence of ER-negative breast cancer. Among women aged 50 to 69 years, the incidence of ER-positive breast cancers decreased by 12%, compared to a 4% decrease in ER-negative breast cancer among this age group. These declines were statistically significant. 

Researchers can only indirectly infer the cause of the dramatic decline in ER-positive breast cancers in 2003, occurring most prominently among the older patient population. However, the author stated that it would make sense that with such a large number of women reducing HRT, a decrease in breast cancer incidence may be observed within a short period. 

Reference: Ravdin P, Cronin K, Howlander N, et al. A Sharp Decrease in Breast Cancer Incidence in the United States in 2003. Proceedings from the 2006 annual San Antonio Breast Cancer Symposium (SABCS). San Antonio, Texas. Oral presentation December 14, 2006. Abstract 5. 

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