Pregnancy associated diffuse malignant fibrous mesothelioma of peritoneum. Immunohistochemical studies of ectopic decidual reaction and concomitant myofibroblastic and mesothelial proliferations.

A case of diffuse proliferations in the peritoneum in a 24-year-old woman several months post partum has been reported. Multinodular growth of myofibroblastic cells without atypia was observed, which predominated and revealed a positive immunohistochemical reaction to muscle-specific actin and vimentin, and dispersed clusters of mesothelial cells showing a positive reaction to cytokeratin and vimentin, with moderate atypia and scarce mitotic figures. On cesarean section the greater omentum contained an ectopic decidua in the form of a nodule. Decidual cells, in part transforming into spindle-shaped cells, showed the presence of desmin. Multifocal proliferation of myofibroblasts, most probably of metaplastic type, concomitant with pregnancy-induced peritoneal deciduosis and diffuse progressive mesothelial proliferations has been identified.