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The report from the task force states: “Most women identified as ‘high risk’ will not develop breast cancer, and the majority of breast cancer cases will arise in women who are not identified as having increased risk.” The group also noted that the type of risk calculator generally used is not recommended for women who are known or suspected to have mutations in BRCA genes, which greatly increase the risk of breast cancer.
Dr. Heidi Nelson, a research professor at Oregon Health and Science University in Portland, Oregon, who directed the data analysis used by the task force, said it was an especially difficult dilemma to weigh the risks and benefits of prescribing drugs with significant risks to healthy people.
She and other researchers analyzed multiple studies that found that the drugs could reduce the incidence of invasive cancer by 30 percent to 68 percent, compared with placebos. When the drugs were compared with each other, tamoxifen had a greater protective effect than raloxifene. But it was more likely to cause blood clots. Older women, meaning those over 50, were more likely to develop blood clots from the drugs, and to develop uterine cancer while taking tamoxifen.
Dr. Nelson noted that some studies found women would be more willing to take the drugs if they could truly prevent breast cancer entirely, rather than just lowering the risk, or if the drugs had no side effects.
One thing that might help doctors and patients, she said, is to keep in mind that the adverse effects were more common in older women. In addition, she said, some risk factors seemed to carry more weight than others, and younger women who had had biopsies showing a condition called atypical hyperplasia did seem to be at added risk and might be among the best candidates for taking the drugs. She said the data suggested that a five-year course of treatment could have protective effects that would continue even when the drugs were stopped.