In recent years, the incidence of gastrointestinal system cancers has been steadily increasing. Early diagnosis of gastrointestinal tumors is one of the most important factors in determining disease-specific survival. Another significant factor is managing the treatment process according to the stage of the tumor. The primary goal in diagnosis and treatment is to increase disease-free survival, improve overall survival, and achieve a cure. Undoubtedly, one of the most fundamental elements of this success is the tumor boards and multimodal approaches created through the collaboration of surgical oncology, medical oncology, radiation oncology, pathology, and radiology clinics. With this book, the aim is to address recent developments in the diagnosis and treatment of gastrointestinal system cancers and to evaluate these developments from a multimodal perspective within a single volume, in collaboration with the relevant specialties.