OPPOSITE: Outcome Prediction of Systemic Treatment in Esophagogastric Carcinoma

Patients with locally advanced, resectable gastric or esophagogastric junction adenocarcinoma will receive a biopsy of the primary tumor, followed by standard-of care neoadjuvant systemic treatment; after neoadjuvant therapy tumor biopsies will be taken from different sites of the resection specimen. * Aim 1: Organoid cultures of pre-treatment tumor biopsies will be established and exposed to the same chemotherapy as the corresponding patient; in vitro response to treatment will be correlated with the in vivo response of patients. * Aim 2: Whole genome, methylome and RNA sequencing of tumors biopsies and organoids will be performed prior to as well as after systemic treatment. Histological and clinical outcome will be correlated with molecular subtypes.