Compared with the breast cancer risk women in the general population have, breast cancer survivors have a substantially higher risk of developing a second primary contralateral breast cancer. Adjuvant hormonal therapy reduces this risk, but preliminary data indicate that it may also increase risk of hormone receptor–negative contralateral tumors. We conducted a population-based nested case-control study including 367 women diagnosed with both first primary estrogen receptor (ER)–positive invasive breast cancer and second primary contralateral breast cancer and 728 matched control women diagnosed only with a first breast cancer. Data on adjuvant hormonal therapy, other treatments, and breast cancer risk factors were ascertained through telephone interviews and medical record abstractions. Two-sided statistical tests using conditional logistic regression were conducted to quantify associations between adjuvant hormonal therapy and risk of hormone receptor–specific subtypes of contralateral breast cancer (n = 303 ER+ and n = 52 ER- cases). Compared with women not treated with hormonal therapy, users of adjuvant tamoxifen for 5 years had a reduced risk of ER+ contralateral breast cancer [odds ratio, 0.4; 95% confidence interval (CI), 0.3–0.7], but a 4.4-fold (95% CI, 1.03–19.0) increased risk of ER- contralateral breast cancer. Tamoxifen use for <5 years was not associated with ER- contralateral breast cancer risk. Although adjuvant hormonal therapy has clear benefits, risk of the relatively uncommon outcome of ER- contralateral breast cancer may now need to be tallied among its risks. This is of clinical concern given the poorer prognosis of ER- compared with ER+ tumors. [Cancer Res 2009;69(17):6865–70]
Hormonal therapy with drugs like tamoxifen is one of the most common treatments for breast cancer because it has been shown to reduce the risk of dying from the disease but, as this study now suggests, there are long term risks associated with the treatment.
Thus we see that on one hand, long-term tamoxifen use among breast cancer survivors decreases the risk of developing the most common, less aggressive type of second breast cancer, but on the other, such use is associated with a more than four-fold increased risk of a more aggressive, difficult-to-treat type of cancer known as ER-negative disease in the breast opposite to the initial tumor.
At this stage, it isn't clear what the mechanism for the development of ER-negative contralateral disease is, but it is worrying risk given the poor prognosis associated with the disease.
Li, C., Daling, J., Porter, P., Tang, M., & Malone, K. (2009). Adjuvant Hormonal Therapy for Breast Cancer and Risk of Hormone Receptor-Specific Subtypes of Contralateral Breast Cancer Cancer Research, 69 (17), 6865-6870 DOI: 10.1158/0008-5472.CAN-09-1355