Multiple primary cancers with gastrointestinal malignant tumors as the first manifestation: Three case reports and review of literature.

Background: The incidence of malignant gastrointestinal (GI) tumors is increasing, and advancements in medical care have significantly improved patient survival rates. As a result, the number of cases involving multiple primary cancers (MPC) has also increased. The rarity of MPC and the absence of sensitive and specific diagnostic markers often lead to missed or incorrect diagnoses.