Does Tamoxifen reduce the risk of cancer in a second breast in BRCA1 and BRCA2 carriers?

The risk of breast cancer is high (~80%) in women who inherit a damaged (mutated) gene called BRCA1 or BRCA 2 (‘BRCA carriers’). Following the first diagnosis, their risk of developing another tumor in the second breast within 10 years is 30%.

Some types of breast cancer need estrogen (female sex hormone) to grow. These cancer cells respond to estrogen via its receptor. Tamoxifen (Nolvadex) prevents estrogen from binding its receptor on cancer cells and stops them from growing. Tamoxifen is effective in breast cancer cells that have receptors for the hormones estrogen or progesterone (hormone receptor-positive). The present study tested whether treatment with Tamoxifen protects BRCA carriers (already diagnosed with breast cancer) from getting cancer in the opposite breast.

This study was based on a questionnaire. 285 women with bilateral breast cancer (cancer in both breasts) and 751 women with unilateral cancer (cancer in one breast) were included. These women tested positive for the BRCA1 or BRCA2 gene mutation. Tamoxifen use was compared between patients with bilateral cancer and patients with unilateral cancers. Participants were followed-up for as long as it took for another tumor to occur.

The risk of breast cancer occurrence in the second breast was reduced by more than 50% in carriers of BRCA1 and BRCA2 mutations when Tamoxifen was given as treatment for the initial breast cancer. This effect was similar for women diagnosed before or after menopause.

In conclusion, this study found that Tamoxifen treatment for breast cancer protects women with BRCA1 or BRCA2 mutations from getting cancer in the second breast.

The main benefit of Tamoxifen is in reducing hormone receptor-positive breast cancer. Whether Tamoxifen is effective in reducing recurrence in BRCA1/2 carriers with hormone receptor-negative breast cancers is unclear.