Extended adjuvant endocrine therapy in a longitudinal cohort of young breast cancer survivors.

Extended adjuvant endocrine therapy (eET) improves outcomes in breast cancer survivors. Most studies however have been limited to postmenopausal women, and optimal eET for young survivors is uncertain. We report eET use among participants in the Young Women's Breast Cancer Study (YWS), a multicenter prospective cohort of women age ≤40 newly diagnosed with breast cancer enrolled between 2006-2016. Women with stage I-III hormone receptor-positive breast cancer, ≥6 years from diagnosis without recurrence were considered eET candidates. Use of eET was elicited from annual surveys sent years 6-8 after diagnosis, censoring for recurrence/death. 663 women were identified as eET candidates with 73.9% (490/663) having surveys eligible for analysis. Among eligible participants, mean age was 35.5 (±3.9), 85.9% were non-Hispanic white, and 59.6% reported eET use. Tamoxifen monotherapy was the most reported eET (77.4%), followed by aromatase inhibitor (AI) monotherapy (21.9%), AI-ovarian function suppression (AI-OFS) (6.8%) and tamoxifen-OFS (3.1%). In multivariable analysis, increasing age (per year odds ratio [OR]: 1.10, 95% confidence interval [CI]: 1.04-1.16), stage (II v. I: OR: 2.86, 95% CI: 1.81-4.51; III v. I: OR: 3.73, 95%CI: 1.87-7.44) and receipt of chemotherapy (OR: 3.66, 95% CI: 2.16-6.21) were significantly associated with eET use. Many young breast cancer survivors receive eET despite limited data regarding utility in this population. While some factors associated with eET use reflect appropriate risk-based care, potential sociodemographic disparities in uptake warrants further investigation in more diverse populations.