Development and validation of RSClin N+ tool for hormone receptor-positive (HR+), HER2-negative (HER2-), node-positive breast cancer.

508 Background: The RSClin tool was developed to provide estimates of recurrence risk and absolute chemotherapy benefit for patients with HR+/HER2- node-negative breast cancer using the 21-gene recurrence score (RS) and clinicopathologic factors. For patients with node-positive disease, we developed a new tool (RSClin N+) to increase the prognostic and predictive utility of risk estimates by combining RS with clinicopathologic factors and menopausal status. Methods: We used Cox regression to estimate 5-year risk of invasive disease or death (iDFS) and likelihood ratio (LR) tests to compare fit of RSClin N+ with RS alone and clinicopathologic factors (tumor grade, tumor size, positive lymph nodes [N1/N2], age) alone in 5283 node-positive patients treated with chemoendocrine therapy (CET) vs endocrine therapy alone (ET) in the S8814 (n=367: N1=227, N2=140, all postmenopausal) and RxPONDER (n/N1=4916) trials. Data from both studies were pooled and stratified by study for postmenopausal models; premenopausal models were fit only on RxPONDER data. Absolute CET benefit across combination of covariate values was estimated as the difference between ET and CET risk estimates. Validation of RSClin N+ was performed in 592 Nmic/N1 patients in the Clalit registry real-world dataset. Results: For pre- and postmenopausal patients, RSClin N+ provided significantly more prognostic information for iDFS than RS or clinicopathologic factors alone (LR Chi-square p<0.05). In postmenopausal patients, RS and clinicopathologic factors were independently prognostic, but only RS showed interaction with CET benefit (p=0.016). Absolute CET benefit for iDFS ranged from <0.1% to 21.5% as the RS increased from 0 to 50. Table shows examples of risk stratification and predicted benefit by RS within clinical low and high risk patients. In premenopausal patients, both RS and clinicopathologic factors remained prognostic but no interaction was observed between RS and CET.In external validation, RSClin N+ risk estimates were concordant with (Lin’s concordance=0.92) and prognostic for observed risk (HR 1.75; 95% CI 1.38 to 2.20). Conclusions: The RSClin N+ model provides improved estimates of prognostic risk and absolute CET benefit than clinical or genomic data alone in node-positive, HR+/HER2- breast cancer and could be used in patient counseling. [Table: see text]