Primary tumors of the exocrine pancreas. Classification, overview, and recent contributions by immunohistochemistry and electron microscopy.

Histologic classification and subtyping of primary tumors of the exocrine pancreas have great prognostic value, although most tumors--such as duct cell adenocarcinomas--are consistently high grade with respect to biologic behavior. The mortality rate for these tumors is nearly equal to the incidence rate, and prognosis does not appear to be significantly influenced by histologic grade of the primary tumor. Other tumors, such as solid and papillary epithelial neoplasms, have intermediate to low malignant potential, while microcystic adenomas have proved to be essentially benign. Recent application of immunohistochemistry, along with electron microscopy, has helped to elucidate the histogenesis of some tumor subtypes. An intestinal phenotype has been demonstrated for mucinous cystic neoplasms with goblet cells, Paneth cells, and a variety of endocrine cells. Divergent tumor differentiation has been seen in a rare group of mixed tumors with variable composition of duct, acinar, or endocrine cells. Some infantile pancreatic tumors, known as pancreatoblastomas, may also show mixed-cell composition. Immunohistologic studies of solid and papillary epithelial neoplasms have not yet identified a consistent or reliable phenotype or marker, and ultrastructural studies have reached disparate conclusions regarding histogenesis. The rarity of other tumors--e.g., those with osteoclasttype giant cells--has hampered complete characterization.