Updated SEER database study of 21-gene assay to assess breast cancer–specific mortality and benefit of chemotherapy by race and ethnicity.

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533 Background: In randomized studies (NSABP B-20, SWOG 8814, TAILORx, RxPONDER), the predictive effect of the 21-gene Breast Recurrence Score assay (RS) on chemotherapy (CT) benefit has been evaluated in patients with node-negative (N0) and node-positive (N+) breast cancer. Retrospective analyses of TAILORx and RxPONDER demonstrated differences in outcome by racial/ethnic groups (RG); however, there were no differences in CT benefit for any of the clinical outcome measures. In this study we characterized the association between the RS result and breast cancer-specific mortality (BCSM) and chemotherapy benefit in BCSM in RG in the recently updated, population-based SEER registries. Methods: Eligible patients were from the SEER 17 registries, November 2022 submission, and had non-metastatic, hormone receptor-positive, HER2-negative, node negative (N0) breast cancer diagnosed between 2006 and 2019 with a RS result. Analyses were weighted using the inverse of estimated propensity of using CT (yes vs. none/unknown). Prognosis was ascertained by estimating breast cancer specific survival (BCSS) at 9 years among patients with CT recorded as none/unknown by RG. The log-rank test was used to compare BCSM by RG. Multivariable Cox models, including RS result (26-100 vs. 0-25) x CT interaction, were used to assess whether RS result was predictive of CT benefit. Analyses were performed in the overall cohort and in Hispanic (H), non-Hispanic Asian and Pacific Islander (NHAPI), non-Hispanic Black (NHB), and non-Hispanic White (NHW) RGs. Results: There were 145,642 eligible patients and 3,212 breast cancer deaths. Median follow-up was 68 months. NHB patients had higher median RS result and CT usage than other groups (p < .001). In N0 patients, higher RS result was associated with lower 9-year BCSS, both overall and by race/ethnicity (p < .001). In Cox multivariable models (Table), the interaction between RS result by CT was significant among all N0 patients (p < .001) and in NHB patients (p = 0.005), NHW patients (p = 0.002), and H patients (p = 0.019), but not in NHAPI patients (p = 0.766). Analyses of patients with N+ disease and more detailed analysis of CT benefit according to RS results by RG are underway and will also be presented. Conclusions: Real-world evidence from the SEER registries in over 145,000 patients confirms the 21-gene assay is prognostic for BCSS in all groups by race and ethnicity. The RS result was predictive of CT benefit in N0 patients overall and in Hispanic, NHB, and NHW patients, but not in NHAPI patients in this analysis. [Table: see text]