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Background: The optimal treatment strategy after first-line induction therapy in advanced HER2-positive oeso-gastric adenocarcinoma (OGA) remains challenging. Methods: Patients treated with trastuzumab (T) plus platinum salts and fluoropyrimidine (F) as first-line chemotherapy between 2010 and 2020 for HER2-positive advanced OGA at 17 academic care centers in France, Italy, and Austria were included. The primary objective was the comparison of F + T vs T alone as maintenance regimen in terms of progression-free survival (PFS) and overall survival (OS) after a platinum-based chemotherapy induction + T. As secondary objective, PFS and OS between patients treated with reintroduction of initial chemotherapy or standard second-line chemotherapy at progression were assessed. Results: Among the 157 patients included, 86 (55%) received F + T and 71 (45%) T alone as a maintenance regimen after a median of 4 months of induction chemotherapy. Median PFS from start of maintenance therapy was 5.1 months in both groups (95% CI 4.2–7.7 for F + T and 95% CI 3.7–7.5 for T alone; p = 0.60) and median OS was 15.2 (95% CI 10.9–19.1) and 17.0 months (95% CI 15.5–21.6) for F + T and T alone, respectively (p = 0.40). Of 112/157 patients (71%) receiving systemic therapy after progression under maintenance, 26/112 (23%) were treated with a reintroduction of initial chemotherapy + T and 86/112 (77%) with a standard second-line regimen. Here, median OS was significantly longer with the reintroduction (13.8 (95% CI 12.1–19.9) vs 9.0 months (95% CI 7.1–11.9); p = 0.007) as confirmed by multivariate analysis (HR 0.49; 95% CI 0.28–0.85; p = 0.01). Conclusion: No additional benefit of adding F to T monotherapy as a maintenance treatment could be observed. Reintroduction of initial therapy at first progression may be a feasible approach to preserve later treatment lines. [ABSTRACT FROM AUTHOR]