Comparison of Molecular-Genetic Concordance of the Primary Tumor and Brain Metastases of Gastroesophageal Cancers

Inclusion Criteria: 1. Men and women aged 18 years or older included in the GASTROBRAIN study with available clinicopathological and treatment data. 2. Histologically confirmed gastric adenocarcinoma, esophageal carcinoma (adenocarcinoma or squamous cell carcinoma), or gastroesophageal junction adenocarcinoma. 3. History of neurosurgical resection of brain metastases with available archival tissue material, and histological confirmation of metastatic lesion originating from gastroesophageal cancer. 4. Availability of paired FFPE tissue samples from primary tumor and matched brain metastasis. Exclusion Criteria: 1. Synchronous or metachronous multiple primary malignancies involving sites other than the stomach, esophagus, or gastroesophageal junction. 2. Primary tumor located outside the gastrointestinal tract. 3. Histologically confirmed non-epithelial gastrointestinal malignancy (e.g., neuroendocrine tumors, sarcoma, gastrointestinal stromal tumor, lymphoma). 4. Intact brain parenchyma (e.g., metastases confined to skull bones or soft tissues of the head without brain parenchymal involvement). 5. Insufficient tumor material or poor sample quality for molecular analysis, defined as: * Tumor cell content \< 70% in the area of microdissection, or * Significant nucleic acid degradation compromising molecular testing 6. Missing one sample from a paired set (either primary tumor or brain metastasis unavailable).