Tamoxifen May Reduce Contralateral Breast Cancer Risk

Adjuvant endocrine therapy with tamoxifen or aromatase inhibitors may reduce the risk for contralateral breast cancer (CBC) in women with invasive breast cancer according to results from a study published in JAMA Oncology.

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Even after completing treatment, approximately 5% of patients with breast cancer will develop CBC within 10 years of initial diagnosis. Some studies have shown that tamoxifen citrate and aromatase inhibitors may reduce CBC risk, but little is known about this relationship in real-world clinical management settings, where varying durations of and gaps in treatment are common.  

Therefore, researchers led by Gretchen L Gierach, PhD, MPH, National Institutes of Health (Bethesda, MD), conducted a retrospective study to determine whether adjuvant treatment with tamoxifen and aromatase inhibitors could be used to reduce CBC risk within a general community setting.

Data from 7541 women diagnosed with primary unilateral invasive breast cancer between January 1990 and December 2008 were included in the study. The researchers analyzed patients from 1 year after initial diagnosis until the development of CBC, another second cancer, death, or last tumor registry follow-up.  

At 6.3 years follow-up, more than half of the study population (52%) used tamoxifen for a medium duration of 3.3 years. Factors associated with tamoxifen use included non-Hispanic white race, age younger than 50 years at diagnosis, stage II disease at diagnosis, estrogen receptor–positive disease, and moderately differentiated grade. Additionally, 25.6% of patients received aromatase inhibitors, with 966 patients receiving them in addition to tamoxifen.

A total of 248 patients developed CBC during the study period, but incidence was much less common among those who received endocrine therapy compared with those who did not (51.2% vs 65.3%). Further, researchers observed an inverse association between decreased CBC risk and increased duration of endocrine therapy. Current tamoxifen users also had a relative risk for CBC of 0.76 per year of use, with an approximately 66% risk reduction for 4 years of therapy compared with nonusers. Risk reductions were slightly smaller for past users but were still significant at least 5 years after stopping tamoxifen therapy

From these results, researchers concluded that tamoxifen therapy was associated with reduced CBC risk during treatment and after its cessation, with risk progressively decreasing as tamoxifen therapy duration increased.