Gastric adenocarcinoma with chief cell differentiation

A case of adenocarcinoma with chief cell differentiation, a novel entity in the stomach, is presented. An 82‐year‐old woman who had undergone distal gastrectomy, was scheduled for upper gastrointestinal endoscopy to clarify mechanical ileus. A protruding tumor 16 × 14 × 9 mm in size was found in the cardia of the remnant stomach. Histological examination indicated a well‐differentiated tubular adenocarcinoma composed of basophilic columnar or cuboidal cells with occasional coarse eosinophilic granules. Immunohistochemical analysis revealed strong expression of pepsinogens I and II and Runt‐related transcription factor gene 3 (RUNX3), characteristic for chief cells, and MUC6 typical for mucous neck cells. However, the tumor cells were negative for the proton pump alpha subunit, a marker for parietal cells. Cdx2 and defensin‐5 were not present, confirming the lack of an intestinal phenotype. The cancer cells shared characteristics of a chief cell and a mucous neck cell, resembling an ancestor of these two cell types, so‐called ‘primitive chief cell’ in fundic gland. In line with these data, the cancer was diagnosed as an adenocarcinoma with chief cell differentiation.