According to the results of a study published in the Journal of the National Cancer Institute, the risk of blood clots among women participating in a breast cancer prevention study was linked with tamoxifen use and body mass index, but not with two gene mutations that influence blood clotting.

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 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, and study participants and their physicians were informed of what treatment the participants had been receiving.

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Although tamoxifen clearly reduced the risk of developing breast cancer, women who took tamoxifen were more likely to develop blood clots. To further explore the predictors of blood clots among study participants, researchers evaluated the importance of two gene mutations that influence blood clotting. An important question was whether the presence of these mutations could predict which women were at greatest risk of tamoxifen-related blood clots. If this proved to be the case, women could be screened for these mutations before starting to take tamoxifen.

  • Among all study participants, women who took tamoxifen were 90% more likely than women who took placebo to develop a blood clot.
  • In a subset of study participants (76 women with a blood clot and 295 women without a blood clot), the risk of blood clots was linked with high body mass index (a comparison of weight to height), but not with either of the two gene mutations that were evaluated.

The researchers conclude that the risk of blood clots among participants in the NSABP Breast Cancer Prevention Trial was linked with tamoxifen use and body mass index, but not with either of two gene mutations that influence blood clotting. The researchers note that there does not appear to be a reason to screen women for these gene mutations before using tamoxifen for breast cancer prevention.

Reference: Abramson N, Costantino JP, Garber JE, Berliner N, Wickerham DL, Wolmark N. Effect of Factor V Leiden and Prothrombin G20210-A Mutations on Thromboembolic Risk in the National Surgical Adjuvant Breast and Bowel Project Breast Cancer Prevention Trial. Journal of the National Cancer Institute. 2006;98:904-10.