Composite gastric carcinoma. Report of a tumor of the carcinoma-carcinoid spectrum.

A composite tumor of the gastric antrum composed of areas of adenocarcinoma and poorly differentiated carcinoid is described, with histochemical and immunohistochemical documentation of endocrine and nonendocrine differentiation. The neoplasm maintained a composite architecture both within the site of origin and in lymph node metastases. The adenocarcinoma displayed a predominantly tubular architecture with focal sheets of clear cells. The poorly differentiated carcinoid was argyrophilic, chromogranin immunoreactive, and focally serotonin immunoreactive, and contained keratin intermediate filaments in a distinctive distribution suggestive of endocrine differentiation. Among the tumors of the carcinoma-carcinoid spectrum, the composite tumor is less common than either carcinomas with interspersed endocrine cells or carcinoids with interspersed nonendocrine cells.