Targeted therapies offer promise to improve oncologic outcomes in esophagogastric adenocarcinoma (EGA). The landmark 'Trastuzumab for gastric cancer (ToGA)' trial established the therapeutic value of Human Epidermal Growth Factor Receptor 2 (HER2) directed therapy in advanced gastric and esophagogastric junction adenocarcinoma, setting a standard of care. Further studies, such as DESTINY-gastric01 and DESTINY-gastric02, have demonstrated the efficacy of antibody-drug conjugates (ADCs) targeting HER2-positive gastric and junctional tumours. The use of HER2-directed therapies in the curative intent setting has more recently been evaluated with favourable outcomes in phase II studies. However, data regarding the prevalence and prognostic significance of HER2 overexpression among patients undergoing treatment with curative intent are limited. Furthermore, few studies have evaluated the clinical significance of intratumoural and tumour-metastatic heterogeneity of HER2 expression, and the finding of HER2-low, in this context, which may have important implications for implementation of neoadjuvant targeted therapies in future. While limited single centre series have evaluated the clinicopathologic significance of HER2 status in EGA, no previous international multicentre study of this nature has been reported.

The goal of this international, multi-center, retrospective observational study is to elucidate the prevalence and clinical significance of HER2 expression in patients with EGA across different regions globally. The study also aims to assess the clinical significance of HER2 heterogeneity in a large, international cohort of patients with EGA, and its association with clinicopathological outcomes in patients with advanced and recurrent disease.

Inclusion Criteria:

• Patients with newly diagnosed EGA from 2000 to April 2024 across all stages of disease
• Primary tumour HER2 status tested as routine in all patients with EGA
• Patients aged 18 years or over

Exclusion Criteria:

- Patients with esophageal squamous cell carcinoma

This study is a multi-center, retrospective, international cohort study including a population of patients diagnosed with EGA from 2000 to April 2024 across all stages of disease with primary tumour routinely tested for HER2 status.