Breast cancer rates fell sharply in 2003, and the lower rate remained in 2004, researchers are reporting today.
The finding, they say, fits with a hypothesis they advanced last December when they had data only from 2003. At that time, national data showed that breast cancer rates fell by nearly 15 percent in the 18 months from July 2002 through December 2003.
The most likely reason for the fall in rates, the investigators, led by Donald A. Berry of M. D. Anderson Cancer Center in Houston, said, was that large numbers of women stopped taking hormone therapy for menopause. And that therapy, a combination of estrogen and progestins, can increase the incidence of breast cancer.
Now, with the 2004 data, the researchers say the effect is less likely to be an anomaly. If rates had gone up again, they explain, it would have meant that their hypothesis was incorrect.
The analysis relied on statistical sleuthing, said Dr. Berry, head of the division of quantitative sciences at M.D. Anderson and senior author of the paper published today in The New England Journal of Medicine.
Over all, in 2003 and 2004, there were nearly 10 percent fewer breast cancer cases than expected. It is the first substantial drop in breast cancer incidence in more than a quarter century. And it involved women age 50 and older, not younger women, and nearly all the decline was in the common type of breast cancer, fed by estrogen, the so-called estrogen-receptor-positive tumors.
The hormone connection came because the Womens Health Initiative, a large federal study examining the health effects of Prempro, the most popular drug prescribed for menopause, was halted in July 2002. The study found that women taking Prempro had an increased risk of heart disease, rather than protection from it. And the presumed benefit of taking the drug was to prevent heart disease. In addition, there was more breast cancer among women taking Prempro than those taking a placebo for comparison. Immediately, sales of Prempro, made by Wyeth, plummeted, falling by 50 percent, and they continued to fall slightly in 2004.
The drop in breast cancer followed immediately.
Those are the facts, said Dr. Peter Ravdin, an oncologist and Dr. Berrys colleague. We think there is a likely connection between them.
Even so, Dr. Ravdin said, hormone therapy does not cause most of breast cancer in the United States. Some cancers do not need estrogen to grow. Others that depend on estrogen grow whether or not a woman is taking Prempro or a similar drug; they are fueled by the estrogen in a womans body.
Dr. Ravdin added that he thought the current guidelines, advising women to use hormone therapy for as short a time as possible for the relief of menopause symptoms, were appropriate.
Dr. Joseph Camardo, senior vice president for global medical affairs for Wyeth Pharmaceuticals, urged caution in interpreting the data. Just because one event follows another does not mean one event caused the other, he pointed out.
“I dont think anyone involved can say they have a specific alternative explanation,” Dr. Camardo said, “but I think other factors should be explored. For example, he said, mammogram use or changes in diet or other drug use might be contributing.”
But any alternative explanation must also take into account the fact that the drop in breast cancer rates was almost entirely estrogen-fed tumors, Dr. Ravdin said.
Dr. Berry said the researchers were well aware of the limitations of their analysis and never said they proved that declining hormone use led to 10 percent less breast cancer.
“Of course, were not sure. We never are,” Dr. Berry said. But it fits. Its a smoking gun.