Drop in breast cancer may reflect decline in hormone use
The sharpest decline in the number of new breast cancer cases ever observed in a single year occurred between 2002 and 2003. That same year, hormone therapy prescriptions took a nosedive following the abrupt termination — for safety reasons — of the Women's Health Initiative (WHI) trial of combined estrogen and progestin (Prempro) in postmenopausal women. Coincidence? Probably not, according to research led by scientists at M.D. Anderson Cancer Center in Houston. The researchers first speculated about a link between decreased breast cancer incidence and falling hormone therapy use in December 2006. They published a more complete set of findings in the April 19, 2007, issue of the New England Journal of Medicine, showing that the rate of new breast cancers remained low in 2004, an indication that the earlier decline was more than just a statistical fluke. Hormone therapy use followed a similar pattern.
The Texas researchers found that breast cancer incidence fell by nearly 7% in 2003, compared with 2002. Using additional calculations, they determined that the decline actually began in mid-2002, after the WHI found that the women in the trial who were taking Prempro had an increased risk of heart disease, breast cancer, stroke, and blood clots, compared with those taking a placebo. The researchers also found that by the end of 2002, prescriptions for HRT had dropped by nearly 40% from the previous year
When they compared the incidence rates of 2001 with those of 2004, the researchers found that the drop in new breast cancers occurred only among women ages 50 and over — in other words, those most likely to have been taking hormone therapy. Moreover, the decrease was almost entirely among women whose tumors need estrogen to grow (known as estrogen receptor–positive, or ER+, tumors)— further evidence that declining hormone use may have something to do with the drop in breast cancer rates.
However, some researchers argue that other factors are involved. For example, a new study by scientists with the American Cancer Society (ACS) points out that breast cancer incidence actually began falling in 1999. The ACS researchers caution that the drop in numbers may be more apparent than actual: Fewer cases of breast cancer are being diagnosed, they fear, because of a leveling-off of the number of women getting mammograms. The Houston researchers recognize this possibility, but note that a drop in screening should have a comparable impact on the number of ER+ and ER− tumors, which was not what they found.
Others have voiced doubts that stopping hormone therapy could have such a profound impact so quickly. The authors of the New England Journal of Medicine study suggest that it can, citing anecdotal reports of breast cancer tumors actually shrinking within weeks of stopping hormone therapy. Nevertheless, they concede that the relationship between hormone therapy and breast cancer is complex and needs further elucidation. More information is expected in 2007.
September 2007 update404: