U.S. breast cancer rates dropped 7 percent in 2003 after millions of women stopped taking hormone drugs for symptoms of menopause because the drugs had been linked to an increased risk of developing the disease, researchers said Thursday.
The sharp fall was in the type of breast cancer that needs the hormone estrogen to grow, which accounts for 70 percent of all cases of the disease. The decline was the largest in three decades, said researchers at the San Antonio Breast Cancer Symposium.
A study funded by the National Cancer Institute and the M.D. Anderson Cancer Center in Houston concluded that 14,000 fewer women were diagnosed with breast cancer in 2003 than in 2002. More than 200,000 new cases of breast cancer are diagnosed each year.
The study doesn’t prove a causal link between the drop in hormone usage and the slide in breast cancer rates, but co-author Dr. Rowan Chlebowski, of Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center, said it was the only plausible explanation.
There was a 1 percent drop in mammograms in 2003, but it would not account for the large decline in cancer diagnoses, he said.
“To have this kind of drop in cancer rates, you need something very big to explain it,” he said.
The results are in line with a study reported last month that found the incidence of breast cancer among patients in Kaiser Permanente’s Northern California region fell 10 percent between 2001 and 2003, a period during which hormone replacement therapy dropped 68 percent. That report, published in the Journal of Clinical Oncology, found that breast cancer rates throughout California fell 11 percent during the same period and continued to decline in 2004.
Taken together, the studies provide “strong supporting evidence” of the link between hormone therapy and breast cancer, said Lisa J. Herrinton, a Kaiser Permanente epidemiologist and co-author of the earlier study.
Prescriptions for the hormones estrogen and progestin fell by almost half after government researchers concluded in mid-2002 that the drugs increased the risk of breast cancer in postmenopausal women.
The latest study found that breast cancers fueled by estrogen fell 8 percent in 2003. In women ages 50 to 69, new cases of estrogen-related breast cancer fell 12 percent.
Dr. Peter Ravdin of M.D. Anderson, a co-author of the study, said it was not clear whether discontinuation of hormone therapy completely stopped very small cancers from growing or merely slowed them down so they remained undetectable.
For that reason, Dr. Julia Smith of New York University Cancer Institute said it was too soon to conclude that breast cancer rates had turned a corner.