Rodríguez, July

e20761 Background: The prognostic role of serial circulating tumor DNA (ctDNA) dynamics during osimertinib treatment has been described mainly in Asian and European populations; however, it remains poorly defined in Hispanic patients with advanced EGFR-mutant non–small cell lung cancer (NSCLC). Our aim was to determine whether serial plasma EGFR testing identifies patients at high risk of early mortality. Methods: This multicenter, retrospective study included 64 Hispanic patients with stage IIIB–IV EGFR-mutant NSCLC treated with first-line osimertinib between 2018 and 2023. Tumor tissue genotyping was performed using hybrid-capture next-generation sequencing. Plasma EGFR mutations were assessed at baseline, week 8, and week 24. Overall survival (OS) and progression-free survival (PFS) were analyzed using Cox models adjusted for p53 status, RAF-pathway alterations, and metastatic burden. Results: The cohort had a median age of 61 years, with 65.6% of patients being female and 56.2% being never-smokers. Sensitizing EGFR mutations consisted mainly of exon 19 deletion (59.4%) and L858R (39.1%). Relevant co-mutations were identified in TP53 (29.7%) and RAF-pathway genes (6.2%). Plasma ctDNA remained positive in 42.2% of patients at week 8 and 22.0% at week 24. Week-8 ctDNA persistence was strongly associated with an inferior OS (HR 9.36, 95% CI 3.80–23.03, p < 0.001) after multivariable adjustment. Week-24 positivity also predicted worse OS (HR 3.34, 95% CI 1.13–9.89, p = 0.02). ctDNA status at week 8 or 24 was not significantly associated with PFS. Patients with TP53 or RAF pathway mutations demonstrated higher rates of molecular persistence. Conclusions: In Hispanic patients receiving Osimertinib, ctDNA positivity at 8-24 weeks is able to identify patients with poor survival outcomes. ctDNA monitoring may provide an early, non-invasive biomarker to guide treatment intensification and trial enrollment in EGFR-mutant NSCLC.