Ectopic decidua mimicking metastatic lesions--report of three cases and review of the literature.

The development of ectopic decidua is generally considered as a physiologic phenomenon of pregnancy, even though it has been occasionally noted in non-pregnant women. We present three cases of ectopic decidua located in the greater omentum and discuss the histopathologic characteristics, as well as the problems encountered in the differential diagnosis from metastatic lesions. Decidual transformation of the omentum could simulate, both grossly and microscopically, malignant neoplastic lesions such as malignant deciduoid mesothelioma, metastatic mucin-producing adenocarcinoma or even metastatic squamous carcinoma. The application of immunohistochemistry is essential for the establishment of the diagnosis, especially in the presence of deceptively atypical vacuolated cells resembling carcinoma cells. Ectopic decidua does not require any therapeutic intervention because it usually regresses postpartum without any complications.