This report highlights a diagnostically challenging case of diffuse deciduoid mesothelioma occurring in the peritoneum of a 25-year-old woman, 8 months postpartum. Optimally debulked tumor consisted of sheets of polygonal cells arranged in solid, trabecular, and pseudopapillary configurations, with vesicular, occasionally grooved nuclei and small nucleoli. A barrage of immunohistochemical stains revealed an unusual staining pattern characterized by diffusely positive keratin, WT-1, and mesothelin staining, but lack of calretinin positivity. Electron microscopy demonstrated only rare thin, long, branching cellular projections. Cytogenetics revealed balanced translocations of 12p and 1q and 16p. Based on compiled ancillary studies, a diagnosis of deciduoid mesolthelioma was made and supported by consultations from experts at 3 outside facilities. Twenty-seven months after diagnosis, the patient is alive and undergoing treatment with progression of disease. This case is presented in detail, and a discussion of the diagnostic criteria and current application of those criteria is provided.
Keywords: deciduoid; mesothelioma; peritoneal mesothelioma.