Signet-ring cell carcinoid: a primary hepatic carcinoid tumor with cytoplasmic inclusions comprising of aggregates of keratin.

A case of primary hepatic carcinoid tumor was recently encountered, which was argyrophil and showed positive reactions to serotonin, gastrin and pancreatic peptide in an immunohistochemical hormonal study. The tumor had unusual morphologic features. The neoplastic cells had a signet-ring cell appearance, similar to the signet-ring cells normally seen in mucin-producing adenocarcinoma. Ultrastructural and immunohistochemical studies revealed the formation of the signet-ring cells to have been caused by the presence of cytoplasmic inclusions consisting of cytokeratins. Further investigation, using eight monoclonal antibodies recognizing cytokeratins of different molecular weights, showed the accumulated cytokeratins to be of low and medium molecular weights. The morphologic observations in this unusual case of hepatic carcinoid tumor are described and reported cases with similar features, for which we propose the term "signet-ring cell carcinoid," are reviewed.