Deciduoid mesothelioma in the pelvic cavity.

A very rare case of deciduoid mesothelioma in the pelvic cavity is presented. A 24-year-old woman (gestational stage: 28 weeks and 6 days) was admitted because of a tumor mass in the abdominal cavity. A well-circumscribed and fibrously encapsulated tumor mass was revealed in the Douglas cavity. Histologically, tumor cells were arranged in a solid sheet with deciduoid appearance and showed partial glandular and papillary structures. The tumor cells contained PAS positive and diastase-digested granules in the cytoplasm as well as alcian-blue positive and hyaluronidase-digested substances in the stroma. The cellularity of the tumor cells was moderate and mitoses were rare. There was partial tumor necrosis and tumor cells had infiltrated through the fibrous capsule. Immunohistochemically, the tumor cells were reactive for pancytokeratin, cytokeratin5/6, vimentin, HBME-1, calretinin and thrombomodulin. Ultrastructurally, numerous, long microvilli, tonofilaments and desmosome junctions could be seen. Consequently, this case was diagnosed as deciduoid mesothelioma and 2 years and 4 months after operation, the patient's clinical course has been good. This case is considered to be the first reported in Japan.