Background: A growing body of evidence suggests that nitrogenous bisphosphonates may reduce the risk of developing a first breast cancer and may prevent metastases among breast cancer survivors. However, their impact on risk of second primary contralateral breast cancer is uncertain.
Methods: Within a nested case-control study among women diagnosed with a first primary estrogen receptor-positive invasive breast cancer at ages 40-79 years, we assessed the association between post-diagnostic bisphosphonate use and risk of second primary contralateral breast cancer. We used multivariable-adjusted conditional logistic regression to estimate odds ratios (ORs) and 95% confidence intervals (CIs) comparing 351 contralateral breast cancer case subjects with 662 control subjects (ie, breast cancer patients not diagnosed with contralateral breast cancer) who were incidence density-matched on county; race/ethnicity; and age at, year of, and stage at first breast cancer diagnosis. We performed sensitivity analyses with respect to bisphosphonate type and confounding by indication. All statistical tests were two-sided.
Results: Current use of any nitrogenous bisphosphonate and use specifically of alendronate were both associated with reduced risks of contralateral breast cancer compared with never use (OR = 0.41, 95% CI = 0.20 to 0.84 and OR = 0.39, 95% CI = 0.18 to 0.88, respectively). The risk of contralateral breast cancer further declined with longer durations of bisphosphonate use among current users (P(trend) = .03). Results were similar in analyses restricted to patients with a history of osteoporosis or osteopenia.
Conclusions: Bisphosphonate use was associated with a substantial reduction in risk of contralateral breast cancer. If this finding is confirmed in additional studies, nitrogenous bisphosphonate therapy may be a feasible approach for contralateral breast cancer risk reduction.