LAUSR

555 Background: Identification of N+ breast cancer patients with a limited risk of recurrence improves selection of those for which chemotherapy and/or extended endocrine therapy (EET) may be most appropriate to reduce overtreatment. BCIN+ integrates gene expression with tumor size and grade, and is highly prognostic for overall (0-10yr) and late (5-10yr) distant recurrence (DR) in N1 patients. Clinical Treatment Score post-5-years (CTS5) is a prognostic model based on clinicopathological factors (nodes, age, tumor size and grade) and significantly prognostic for late DR. The current analysis compares BCIN+ and CTS5 for risk of late DR in N1 patients. Methods: 349 women with HR+, N1 disease and recurrence-free for ≥5 years were included. BCIN+ results were determined blinded to clinical outcome. CTS5 was calculated as previously described (Dowsett et al, JCO 2018; 36:1941). Kaplan-Meier analysis and Cox proportional hazards regression for late DR (5-15y) were evaluated. Results: 64% of patients were > 50 years old, 34% with tumors > 2cm, 79% received adjuvant chemotherapy and 64% received up to 5 years of ET. BCIN+ stratified 23% of patients as low-risk with 1.3% risk for late DR vs those classified as high-risk with 16.1% [HR 12.4 (1.7-90.4), p = 0.0014]. CTS5 classified patients into 3 risk groups: 29% of patients as low-risk (4.2% DR), 37% as intermediate-risk (10.6% DR), and 34% as high-risk (22.1% DR) [HR intermediate vs. low: 2.3 (0.7-7.0), p = 0.16; high vs. low: 5.3 (1.8-15.5), p = 0.002]. In a subset of patients who completed 5 years of ET (N = 223), BCIN+ identified 22% of patients as low-risk with a late DR rate of 2.1%, while CTS5 identified 29% and 37% of patients as low- and intermediate-risk with late DR rates of 5.2% and 10.3%, respectively. Conclusions: BCIN+ classified N1 patients into binary risk groups and identified 20% patients with limited risk of late DR ( < 2%) that may be advised to forego EET and its attendant toxicities/side effects. In comparison, CTS5 classified patients into 3 risk groups, with low- and intermediate-risk of late DR of 4-5% and 10%, wherein the risk-benefit profile for extension of endocrine therapy is less clear.