Purpose:

As survival with early-stage, hormone receptor (HR)-positive breast has improved, it is essential to understand the long-term risks of incident comorbidities with different adjuvant endocrine therapy (ET) options.

Methods:

Women treated with tamoxifen and/or an aromatase inhibitor (AI) for stages 1-3, HR-positive/HER2-negative breast cancer from 2000 to 2016 in either of two healthcare systems in the San Francisco Bay Area were included. We considered the following comorbidities: cerebrovascular accidents, congestive heart failure, dementia, depression/anxiety, diabetes mellitus, hyperlipidemia, myocardial infarction, non-alcoholic steatohepatitis, osteoporosis/fracture, peripheral vascular disease, and venous thromboembolism. Cause-specific Cox proportional hazards models were fit to time-to-new-diagnosis for each comorbidity, accounting for death as a competing risk. Hazard ratios (HR) and 95% confidence intervals (CI) for tamoxifen versus AI were reported.

Results:

Among 2,902 analyzed patients, the median age at diagnosis was 58.3 years; 67.6% were non-Hispanic white, 22.3% Asian, 7.5% Hispanic, and 1.7% non-Hispanic Black. Half (54.7%) used AIs only, 27.6% used tamoxifen only and 17.7% used both tamoxifen and AIs sequentially. Tamoxifen was associated with a lower risk of osteoporosis than AI (multivariable HR 0.45, 95% CI 0.32-0.62). No other incident comorbidity risk varied between users of tamoxifen versus AIs.

Conclusion:

In a diverse, multi-institutional, contemporary breast cancer cohort, the only incident comorbidity that differed between ET options was osteoporosis, a known side effect of AIs. These results may inform clinical decision-making about ET, and reassure patients who have bothersome symptoms on AIs that they are unlikely to develop worse comorbidities if they switch to tamoxifen.