Clinical guidelines have recommended high-risk breast cancer patients to extend adjuvant hormone therapy beyond 5 years. However, the prevalence, predictors, and outcomes of extended adjuvant hormone therapy in the real world remain unknown. By linking six Swedish health registries, we prospectively followed 13168 breast cancer patients (2005-2020) from their first prescription of tamoxifen or aromatase inhibitors, and categorized them as extending or not extending adjuvant hormone therapy. Cox regression analysis was used to investigate whether extended therapy was associated with breast cancer outcomes. Among breast cancer patients who were recommended to extend adjuvant hormone therapy by the national guidelines, the proportion of women who extended therapy increased 5 fold during the past 10 years, reaching 80.9% during 2018-2020. Patients were more likely to extend therapy after completing 5-year adjuvant hormone therapy if they were young (< 40 vs ≥ 65 years: OR, 1.71; 95% CI, 1.13-2.58), had positive lymph nodes (OR, 2.25; 95% CI, 1.85-2.73), had high tumor grade (grade 3 vs 1: OR,1.79; 95% CI, 1.34-2.39), received chemotherapy (OR, 5.22; 95% CI, 4.19-6.50), had first-degree relatives who died from breast cancer (OR, 1.84; 95% CI, 1.21-2.81), or had a high income (OR, 1.23; 95% CI, 1.01-1.49). Extended use of adjuvant hormone therapy was statistically significantly associated with improved disease-free survival (HR, 0.72; 95 CI%, 0.55-0.95). This study provides real-world evidence showing the use and improved breast cancer outcomes of extended adjuvant hormone therapy beyond 5 years.

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