Mixed Pancreatobiliary Ductal Adenocarcinoma and Squamous Cell Carcinoma Arising from an Ectopic Pancreas in a Gastric Duplication Cyst—A Rare Double Diagnosis

Gastric duplication cysts (GDCs) are rare congenital anomalies, often identified during infancy or childhood. Although typically benign, there have been sporadic reports of malignant transformations, including adenocarcinoma and rare mixed tumors. Herein, we describe a rare case of mixed pancreatobiliary ductal adenocarcinoma and squamous cell carcinoma occurring within a GDC in a 54-year-old Korean woman with a history of melena and hematemesis. Initial gastroscopy and positron emission tomography-computed tomography (PET-CT) revealed a protruding stomach mass. A laparoscopic total gastrectomy was performed, and histological examination confirmed a mixed carcinoma originating from an ectopic pancreas within the duplication cyst. This case is unique as it is the first reported instance in the world of mixed pancreatobiliary ductal adenocarcinoma and squamous cell carcinoma arising from an ectopic pancreas within a GDC. This highlights the importance of considering pancreatobiliary-type adenocarcinoma in the differential diagnosis of malignancies originating from GDCs, which has implications for treatment strategies.