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Updated results from the National Surgical Adjuvant Breast and Bowel Project Breast Cancer Prevention Trial, published in the Journal of the National Cancer Institute, are similar to initial findings: Among women at high risk of breast cancer, treatment with five years of tamoxifen reduces the risk of breast cancer and increases the risk of endometrial (uterine) cancer and blood clots.

The National Surgical Adjuvant Breast and Bowel Project (NSABP) Breast Cancer Prevention Trial (P-1) began in 1992, with the goal of determining whether tamoxifen (Nolvadex®) could reduce the risk of breast cancer in women at high risk of the disease.

A woman’s risk of breast cancer was estimated on the basis of her age, race, family history of breast cancer, number of births and age at first live birth, age at first menstrual period, number of benign breast biopsies, and diagnosis of atypical hyperplasia. Women were eligible if their five-year risk of breast cancer was at least 1.7%. A total of 13,388 women were randomly assigned to receive either five years of tamoxifen or five years of placebo.

By 1998, there was strong evidence that tamoxifen reduced the risk of breast cancer. At this point, study participants and their physicians were informed of what treatment the participants had been receiving. Although many women in the placebo group subsequently chose to start taking tamoxifen or to enroll in another clinical trial, researchers followed study participants for another seven years. The current study reports on the results of these seven years of follow-up. These findings will most likely be the last published results of this large and important clinical trial.

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The updated results were similar to those from the initial study. Compared to women in the placebo group, women in the tamoxifen group had the following results:

  • 43% reduction in risk of invasive breast cancer
  • 37% reduction in risk of noninvasive breast cancer (DCIS or LCIS)
  • 32% reduction in risk of osteoporotic bone fractures
  • Three-fold increase in risk of endometrial cancer
  • Two-fold increase in risk of pulmonary embolism

The researchers note that for certain subgroups of women, the benefits of tamoxifen treatment are likely to substantially outweigh the risks. A method that has been developed to weigh the risks and benefits may help identify those women who are most likely to benefit from preventive treatment with tamoxifen.

Although other breast cancer prevention clinical trials are currently underway, the researchers note, “Until one of these trials demonstrates a greater net benefit from an alternative therapy, tamoxifen remains the only proven chemopreventive treatment for breast cancer risk reduction.”

Reference: Fisher B, Costantino JP, Wickerham DL et al. Tamoxifen for the Prevention of Breast Cancer: Current Status of the National Surgical Adjuvant Breast and Bowel Project P-1 Study. Journal of the National Cancer Institute. 2005;97:1652-62.