Breast Cancer Index Predicts Extended Endocrine Benefit to Individualize Selection of Patients with HR+ Early-stage Breast Cancer for 10 Years of Endocrine Therapy

PURPOSE: ) breast cancer for extended endocrine therapy (EET) is required to balance modest gains in outcome with toxicities of prolonged use. This study examined the Breast Cancer Index BCI; HOXB13/IL17BR ratio (H/I) as a predictive biomarker of EET benefit in patients from the Investigation on the Duration of Extended Adjuvant Letrozole trial. EXPERIMENTAL DESIGN: BCI was tested in primary tumor specimens from 908 patients randomized to receive 2.5 versus 5 years of extended letrozole. The primary endpoint was recurrence-free interval. Cox models and likelihood ratios tested the interaction between EET and BCI (H/I). RESULTS: = 0.025, respectively). BCI identified approximately 50% of patients with clinically high-risk disease that did not benefit, and with clinically low-risk disease that derived significant benefit, from an additional 2.5 years of EET. CONCLUSIONS: breast cancer.