Real-world impact of trastuzumab-based therapy in HER2-positive advanced gastric cancer: Evidence from a Latin American cancer center.

e16093 Background: Gastric cancer ranks as the fifth leading cause of cancer-related mortality worldwide. HER2-positive cases represent 10–20% of gastric cancers. The addition of trastuzumab to chemotherapy significantly improves survival outcomes; however, real-world data on its effectiveness in Latin American populations remain scarce. Methods: This retrospective study included 22 patients with HER2-positive advanced gastric cancer treated at the Instituto Nacional de Enfermedades Neoplásicas (INEN) from 2019 to 2024. HER2 status was confirmed via immunohistochemistry (IHC) and/or fluorescence in situ hybridization (FISH). Progression-free survival (PFS) was calculated from initiating first-line therapy to disease progression, death, or last follow-up. Survival outcomes were estimated using the Kaplan-Meier method. Results: Among the 22 patients analyzed, HER2 status included n = 15 (68.2%) IHC 3+, n = 4 (18.2%) IHC 2+/FISH-positive, and n = 3 (13.6%) FISH-positive/unknown IHC status. Tumor heterogeneity (defined as < 30% of tumor cells staining positive) was assessed in 13 pathology samples, with 23% classified as heterogeneous and a median HER2 staining percentage of 70% (40%–100%). The median age was 64 (47–82), and 59.1% were male. Tumor locations included the gastroesophageal junction (9.1%), antrum (27.3%), and body (63.6%). Tubular adenocarcinoma was the predominant histology (90.9%), with moderate differentiation (G2) in 77.3%. Metastatic disease was present in 81.8% of patients, with liver (40.9%), non-regional lymph nodes (36.4%), and lungs (18.2%) as common metastatic sites. Seven patients underwent D2 gastrectomy with R0 resection; four remained disease-free (NED). ECOG 1 performance status was observed in 81.8%. Trastuzumab plus CAPOX was the most common first-line regimen (63.6%), followed by Trastuzumab plus modified FOLFOX-6 (31.8%), with a median of 9.5 cycles (2–19). Trastuzumab was administered for a median of 8 cycles (2–17). Nine patients (40.9%) experienced progression or death, with a median follow-up of 9.5 months (1–50). PFS rates at 6, 12, and 36 months were 85%, 65%, and 19%, respectively, with a median PFS of 23 months. Conclusions: The outcomes observed in this cohort were likely influenced by unique factors, including a high prevalence of oligometastatic disease, surgical interventions achieving NED, favorable performance status, and low HER2 heterogeneity. These findings highlight the potential benefits of trastuzumab in this specific subgroup, underscoring the need for further studies to validate these results in Latin American populations.