Background: OFS is an important treatment component for premenopausal women with early and advanced HR+ BC and optimal efficacy may depend on complete E2 suppression with GnRHa, especially when combined with aromatase inhibitors (AI). However, standard assays lack sensitivity and accuracy at very low E2 concentrations, rendering detection of breakthrough ovarian function challenging. More informative data regarding endocrine effects of GnRHa are needed, particularly in young women who derive the most benefit from OFS yet more frequently had breakthrough E2 levels in the SOFT trial substudy, SOFT-EST. Methods: Using a prospective cohort study of women with BC diagnosed at age ≤ 40 years, we identified participants with stage I-IV HR+ BC on OFS treatment with leuprolide, goserelin or triptorelin and blood collected 1 year after diagnosis. Clinical data were obtained through surveys and medical records. Plasma estrogens were measured by liquid chromatography-tandem mass spectrometry, with a limit of detection of 0.2 pg/mL compared to 10 pg/mL with standard assays. We assessed the proportion of women on OFS with E2 > 2.72 pg/mL (10 pmol/L) and > 10 pg/mL. Patient characteristics, estrone (E1) and FSH levels, and disease-free (DFS) and overall survival (OS) were compared between those with and without elevated E2 using Fischer’s exact, Wilcoxon rank sum and log rank tests, respectively. Results: Of 84 patients who met inclusion criteria, 72 (86%) had stage I-III and 12 (14%) had stage IV BC; 25 were on OFS plus AI and 59 were on OFS plus tamoxifen (TAM). Median E2 was 3.1 pg/mL (range 0.2-30.9). 46 patients (55%) had E2 > 2.72 pg/mL, including 8 on AI and 38 on TAM; 4 (5%) had E2 > 10 pg/mL, including 1 on AI and 3 on TAM. Factors associated with E2 > 2.72 pg/mL were no prior chemotherapy, TAM and high E1 (P ≤.05), but not age, BMI or GnRHa schedule (Table). After a median follow-up of 7 years, DFS events were seen in 6 patients with E2 > 2.72 pg/mL and 5 without (P =.232); OS events were seen in 7 patients with E2 > 2.72 pg/mL and 5 without (P =.608). Conclusions: More than half of young women with ER+ BC had E2 > 2.72 pg/mL on OFS, 91% of whom would not have been detected by standard assays. This may be an area amenable to intervention to improve outcomes in young women. However, larger studies are needed to elucidate the optimal E2 target on OFS and clinical implications of incomplete E2 suppression on ultrasensitive assays.

*n = 3 missing data