Initiation of Oral Endocrine Therapy and Survival Benefit Among Women with Early-Stage Breast Cancer

<b><i>Introduction:</i></b> Endocrine therapy (ET) is the cornerstone of systemic treatment for patients with estrogen receptor positive breast cancer, but its uptake and adherence need further improvement. This observational study assessed ET initiation and 1-year adherence and its survival benefit among female Medicare beneficiaries with early-stage breast cancer. <b><i>Materials and Methods:</i></b> This retrospective cohort study analyzed the linked 2011-2019 Surveillance, Epidemiology, and End Results-Medicare data. Female beneficiaries newly diagnosed with hormone receptor positive, stage I-III breast cancer were included. Beneficiaries who initiated tamoxifen, anastrozole, letrozole, or exemestane within 3 months after cancer diagnosis were defined as initiators (<i>n</i> = 24,289), and those who never initiated these treatments were noninitiators (<i>n</i> = 8,899). Adherence was measured using proportion of days covered (PDC) in the continuous 12 months follow-up period. Multivariable logistic regression models were used to assess factors associated with ET initiation and adherence (PDC ≥ 80%), controlling for covariates. Weighted Cox proportional hazard models were used to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) of all-cause and breast cancer related mortality between initiators and noninitiators and by adherence status. <b><i>Results:</i></b> Among eligible female beneficiaries (<i>n</i> = 55,893), 43% initiated ET within 3 months of cancer diagnosis. Among initiators, 77% had PDC ≥ 80% during the first year. Patient's demographics (e.g., older age, race/ethnicity) and baseline health services utilization (e.g., mammography) were associated with ET initiation and adherence. ET initiation and adherence was associated with reduced risk of all-cause (adjusted HR = 0.62, 0.59-0.66; HR = 0.55, 0.53-0.59; respectively) and breast cancer related (adjusted HR = 0.57, 0.50-0.64; HR = 0.41, 0.36-0.47; respectively) mortality compared with noninitiators. <b><i>Conclusion:</i></b> Women with early-stage breast cancer who initiate ET and are adherent to treatment may achieve survival benefits compared with noninitiators.