'Mucin-positive' epithelial mesothelioma of the peritoneum: an unusual diagnostic pitfall

Aims: The histopathological, immunohistochemical and ultrastructural features of a primary 'mucin-positive' epithelial mesothelioma of the peritoneum are reported to draw attention to a potential cause of diagnostic error. Methods and results: Light microscopy showed an infiltrative neoplasm within the gastric wall and omentum which was composed of sheets of 'signet-ring' tumour cells and contained abundant diastase-resistant periodic acid-Schiff-positive material. Immunohistochemistry supported a mesothelial phenotype (cytokeratin AE1/3, thick membranous HBME-1, focal thrombomodulin and calretinin expression and no reactivity for carcinoembryonic antigen, Leu-M1 and Ber-EP4). Ultrastructural features showed large cells with prominent intercellular desmosomes and numerous delicate, elongated microvilli. Within intracytoplasmic neolumina, crystalloidal 'fern-like' bodies were identified with features similar to the ultrastructural appearances of hyaluronic acid crystals. Repeat histochemical analysis following hyaluronidase pretreatment revealed a significantly diminished diastase-resistant periodic acid-Schiff reaction. Conclusions: This case of primary peritoneal 'mucin-positive' epithelial mesothelioma demonstrates morphological and histochemical mimicry with diffuse gastric adenocarcinoma. A similar case has not been previously reported in the peritoneum and an awareness of the tumour (with application of suitable ancillary studies) prevents misdiagnosis and assists in potentially difficult medicolegal cases.