Tamoxifen and AI Can Reduce Long-Term Risk of Contralateral Breast Cancer

Adjuvant therapy in patients diagnosed with breast cancer is known to stall the development of contralateral disease; however, the magnitude and duration of this protective effective is not very clear. To answer this question, researchers at the National Cancer Institute and Kaiser Permanente conducted a retrospective analysis and found that long-term adjuvant treatment with tamoxifen and aromatase inhibitors (AIs) resulted in a proportional decrease in risk of contralateral breast cancer (CBC).

The study cohort included 7541 patients diagnosed with a first primary unilateral invasive breast cancer between January 1, 1990, and December 31, 2008, at a Kaiser Permanente research facility in Oregon or Colorado. Data analysis was from a year after diagnosis through whichever was earlier: CBC diagnosis, diagnosis of a second cancer, death, exit from a Kaiser Permanente healthcare plan, or end of study follow-up. Use of adjuvant tamoxifen or AI was monitored via patient electronic prescription records.

With white women dominating the cohort (92.9%), the median age at initial diagnosis was 60.6 years (range, 24.9 to 84.9 years). During a median 6.3 years of follow-up, 248 women developed CBC, the authors report, 45 of which were in situ and 203 were invasive. However, increased duration of adjuvant treatment with tamoxifen had a significant impact on reducing the risk of CBC: relative risk (RR) per year of tamoxifen treatment was 0.76 (95% CI, 0.64-0.89) with a 66% RR reduction (RR, 0.34; 95% CI, 0.29-0.40) for 4 years of use, compared with nonusers. AI alone also reduced CBC risk (RR for AI users compared with nonusers, 0.48; 95% CI, 0.22-0.97). Women whose primary tumors and CBC expressed estrogen receptor seemed most to benefit from the adjuvant treatment.

Based on their findings the authors conclude that tamoxifen reduced CBC risk during treatment and after cessation, and a longer duration of treatment progressively decreased the risk. “If adjuvant endocrine therapy is indicated for breast cancer treatment, these findings in concert with trial data suggest that women should be encouraged to complete the full course,” they wrote.

Reference

Gierach GL, Curtis RE, Pfeiffer RM, et al. Association of adjuvant tamoxifen and aromatase inhibitor therapy with contralateral breast cancer risk among US women with breast cancer in a general community setting [published online October 7, 2016]. JAMA Oncol. doi: 10.1001/jamaoncol.2016.3340.