Gastric adenocarcinoma of fundic gland type (Chief Cell Predominant Type): Proposal for a new entity of gastric adenocarcinoma

Only a few cases of gastric adenocarcinoma of fundic gland type have been reported. Gastric adenocarcinoma with chief cell differentiation (GA-CCD) has been recently reported as a new variant of gastric adenocarcinoma. However, its clinicopathologic features are uncertain. To elucidate them, GACCDs exhibiting pepsinogen-I expression (10 lesions: Group A) and randomly selected gastric adenocarcinomas of differentiated type (111 lesions: Group B) were evaluated in this study. Cell differentiation by MUC2, MUC5AC, MUC6, CD10, pepsinogen-I, H⁺/K⁺-ATPase and chromogranin A, cell proliferation by Ki-67, and overexpression of p53 protein were evaluated immunohistochemically. In Group A, all GA-CCDs were located in the upper third of the stomach. Tumors were small, with the average maximum diameter ranging from 4 to 20 (average, 8.6) mm. Histologically, GA-CCDs were well-differentiated adenocarcinomas composed of pale gray-blue, basophilic columnar cells with mild nuclear atypia, resembling chief cells. Immunohistochemically, scattered positivity for H ⁺/K⁺-ATPase was observed in addition to expression of pepsinogen-I and MUC6, indicating focal differentiation toward parietal cells. In Group B, pepsinogen-I was very focally expressed in 2 cases. As these 2 cases exhibited different clinicopathological and histologic features, they cannot be categorized as GA-CCD. Mild atypism, no lymphovascular invasion, low proliferative activity, no overexpression of p53, and no recurrence indicated less aggressiveness of GA-CCD. GA-CCD is rare, but it has distinct clinicopathological characteristics, especially in terms of tumor location, histologic features, phenotypic expression, and low-grade malignancy. We propose gastric adenocarcinoma of fundic gland type (chief cell predominant type) as a new entity of gastric adenocarcinoma.