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The New York Times reported on December 15, 2006 that researchers at the MD Anderson Cancer Research Center in Houston, TX have observed that the incidence rates of estrogen-positive breast cancer dropped 15% between August 2002 and December 2003. Estrogen-positive tumors are the most common form of breast cancer and account for 70% of cases of this disease. This observed drop resulted in a 7% drop in breast cancer cases overall, which reverses a general upward trend for breast cancer rates that has been observed since the mid 1940's.
The leading hypothesis to explain the observation is the reduced use of hormones by menopausal women after a large national study associated their use with increased rates of breast cancer. This hypothesis is supported by the observation that most of the observed drop in incidence was observed in women between the ages of 50 and 69. However, although the observed drop in cases is very encouraging and the association between less hormone use is both coincidental and biologically plausible, caution is still urged in interpreting these results because it is based on speculation, not on data:
“Epidemiology can never prove causality,� said Dr. Peter Ravdin, a medical oncologist at the M.D. Anderson center and one of the authors of the analysis. But, he said, the hormone hypothesis seemed to perfectly explain the data and he and his colleagues could find no other explanation.
Everyone who is interested in cancer at both the personal and population levels will be watching to see if this initial observation is the beginning of a trend downward in the incidence of breast cancer. It will also be very useful to see if this observation is made in other populations and if biological causation can be proven.