Esophagogastric Adenocarcinoma

This trial is designed to evaluate a total neoadjuvant approach using D-FLOT as the new standard backbone in patients with resectable esophagogastric adenocarcinoma. It addresses major limitations of current treatment paradigms, builds directly on the strong clinical signal from the MATTERHORN trial, and offers a rational, biologically sound framework for future therapy intensification and innovation. By moving systemic therapy entirely into the preoperative phase, we aim to: * Improve patient outcomes through better adherence and deeper response * Minimize postoperative therapy-related dropout * Create a platform for rational post-surgical drug testing and individualized treatment escalation The trial will provide pivotal evidence to guide the next generation of curative-intent treatment strategies for EGA.

Participants will have a sample of their white blood cells, called T cells, collected using a procedure called leukapheresis. The collected T cells will be sent to a laboratory at Memorial Sloan Kettering to be changed (modified) to become MSLN-targeted CAR T cells, the CAR T-cell therapy that participants will receive during the study. Participant study therapy will take about 3-4 weeks.

Researchers are looking to further our knowledge on disease biology and treatment selection for gastroesophageal adenocarcinoma. The purpose of this study is to see how useful it is to look for changes and characteristics in your genes (molecules that contain instructions for the development and functioning of the cells) and the genes within the tumour. These characteristics may be useful in choosing treatments for patients for the future.